diff --git a/parquet-common/src/main/java/org/apache/parquet/bytes/CapacityByteArrayOutputStream.java b/parquet-common/src/main/java/org/apache/parquet/bytes/CapacityByteArrayOutputStream.java
index 3aa2c736a3..61d92ee87c 100644
--- a/parquet-common/src/main/java/org/apache/parquet/bytes/CapacityByteArrayOutputStream.java
+++ b/parquet-common/src/main/java/org/apache/parquet/bytes/CapacityByteArrayOutputStream.java
@@ -269,20 +269,41 @@ private void writeToOutput(OutputStream out, ByteBuffer buf, int len) throws IOE
       // The OutputStream interface only takes a byte[], unfortunately this means that a ByteBuffer
       // not backed by a byte array must be copied to fulfil this interface
       byte[] copy = new byte[len];
-      buf.flip();
-      buf.get(copy);
+      // Read through a duplicate so the slab's position and limit are left unchanged
+      ByteBuffer content = buf.duplicate();
+      content.flip();
+      content.get(copy);
       out.write(copy);
     }
   }
 
+  /**
+   * Checks that the slabs hold exactly the {@link #size()} bytes that were written. The size is used for the length
+   * prefixes and page headers that describe this content, so serializing content of a different length would
+   * silently produce a corrupt file.
+   */
+  private void checkContentSize() {
+    long contentSize = 0;
+    for (ByteBuffer slab : slabs) {
+      contentSize += slab.position();
+    }
+    if (contentSize != bytesUsed) {
+      throw new IllegalStateException(format(
+          "The slabs of this stream hold %d bytes, but %d bytes were written to it (%d slabs)",
+          contentSize, bytesUsed, slabs.size()));
+    }
+  }
+
   /**
    * Writes the complete contents of this buffer to the specified output stream argument. the output
    * stream's write method <code>out.write(slab, 0, slab.length)</code>) will be called once per slab.
    *
    * @param out the output stream to which to write the data.
    * @throws IOException if an I/O error occurs.
+   * @throws IllegalStateException if the content of the slabs does not match {@link #size()}
    */
   public void writeTo(OutputStream out) throws IOException {
+    checkContentSize();
     for (ByteBuffer slab : slabs) {
       writeToOutput(out, slab, slab.position());
     }
@@ -290,11 +311,16 @@ public void writeTo(OutputStream out) throws IOException {
 
   /**
    * It is expected that the buffer is large enough to fit the content of this.
+   *
+   * @throws IllegalStateException if the content of the slabs does not match {@link #size()}
    */
   void writeInto(ByteBuffer buffer) {
+    checkContentSize();
     for (ByteBuffer slab : slabs) {
-      slab.flip();
-      buffer.put(slab);
+      // Read through a duplicate so the slab's position and limit are left unchanged
+      ByteBuffer content = slab.duplicate();
+      content.flip();
+      buffer.put(content);
     }
   }
 
@@ -349,15 +375,17 @@ public long getCurrentIndex() {
   public void setByte(long index, byte value) {
     checkArgument(index < bytesUsed, "Index: %d is >= the current size of: %d", index, bytesUsed);
 
+    // A slab may be followed by a new one before it is full (see writeInt and writeLong), so the bytes written to
+    // each slab are counted by its position, not by its capacity or limit
     long seen = 0;
     for (int i = 0; i < slabs.size(); i++) {
       ByteBuffer slab = slabs.get(i);
-      if (index < seen + slab.limit()) {
+      if (index < seen + slab.position()) {
         // ok found index
         slab.put((int) (index - seen), value);
         break;
       }
-      seen += slab.limit();
+      seen += slab.position();
     }
   }
 
diff --git a/parquet-common/src/test/java/org/apache/parquet/bytes/TestCapacityByteArrayOutputStream.java b/parquet-common/src/test/java/org/apache/parquet/bytes/TestCapacityByteArrayOutputStream.java
new file mode 100644
index 0000000000..692c20f9d5
--- /dev/null
+++ b/parquet-common/src/test/java/org/apache/parquet/bytes/TestCapacityByteArrayOutputStream.java
@@ -0,0 +1,170 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.apache.parquet.bytes;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
+
+import java.io.ByteArrayOutputStream;
+import java.io.IOException;
+import java.nio.ByteBuffer;
+import java.util.ArrayList;
+import java.util.List;
+import java.util.stream.Stream;
+import org.junit.jupiter.api.AfterEach;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.MethodSource;
+
+/**
+ * Tests that the content serialized by {@link CapacityByteArrayOutputStream} always matches its {@link
+ * CapacityByteArrayOutputStream#size()}, which is used for length prefixes and page headers (GH-3712).
+ */
+public class TestCapacityByteArrayOutputStream {
+
+  private TrackingByteBufferAllocator allocator;
+
+  static Stream<ByteBufferAllocator> allocators() {
+    return Stream.of(new HeapByteBufferAllocator(), new DirectByteBufferAllocator());
+  }
+
+  @AfterEach
+  public void closeAllocator() {
+    if (allocator != null) {
+      allocator.close();
+    }
+  }
+
+  private CapacityByteArrayOutputStream newStream(ByteBufferAllocator baseAllocator, int initialSlabSize) {
+    allocator = TrackingByteBufferAllocator.wrap(baseAllocator);
+    return new CapacityByteArrayOutputStream(initialSlabSize, 1024 * 1024, allocator);
+  }
+
+  private static byte[] writeTo(CapacityByteArrayOutputStream stream) throws IOException {
+    ByteArrayOutputStream out = new ByteArrayOutputStream();
+    stream.writeTo(out);
+    return out.toByteArray();
+  }
+
+  private static byte[] writeInto(CapacityByteArrayOutputStream stream) {
+    ByteBuffer buffer = ByteBuffer.allocate((int) stream.size());
+    stream.writeInto(buffer);
+    assertThat(buffer.hasRemaining()).isFalse();
+    return buffer.array();
+  }
+
+  /**
+   * writeInt and writeLong add a new slab when the current one has fewer bytes left than they need, so a slab may be
+   * followed by another one before it is full. setByte, as used by the RLE encoder to patch a bit-packed run header,
+   * must address the written bytes, not the capacity of the slabs.
+   */
+  @ParameterizedTest
+  @MethodSource("allocators")
+  public void testSetByteAfterPartiallyFilledSlab(ByteBufferAllocator baseAllocator) throws IOException {
+    try (CapacityByteArrayOutputStream stream = newStream(baseAllocator, 8)) {
+      stream.write(new byte[] {0, 1, 2, 3, 4, 5}, 0, 6);
+      // Only 2 bytes left in the first slab: the int goes to a new slab
+      stream.writeInt(0x09080706);
+      stream.write(0);
+      long headerIndex = stream.getCurrentIndex();
+      stream.write(11);
+      stream.setByte(headerIndex, (byte) 10);
+      stream.writeLong(0x1312111009080706L);
+
+      byte[] expected = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 6, 7, 8, 9, 0x10, 0x11, 0x12, 0x13};
+      assertThat(stream.size()).isEqualTo(expected.length);
+      assertThat(stream.getSlabCount()).isGreaterThan(1);
+      assertThat(writeTo(stream)).containsExactly(expected);
+      assertThat(writeInto(stream)).containsExactly(expected);
+    }
+  }
+
+  /** Serializing the content must not change the state of the slabs, so that it can be serialized again. */
+  @ParameterizedTest
+  @MethodSource("allocators")
+  public void testSerializingDoesNotChangeTheContent(ByteBufferAllocator baseAllocator) throws IOException {
+    try (CapacityByteArrayOutputStream stream = newStream(baseAllocator, 4)) {
+      ByteArrayOutputStream expected = new ByteArrayOutputStream();
+      for (int i = 0; i < 100; i++) {
+        stream.write(i);
+        expected.write(i);
+        if (i % 7 == 0) {
+          assertThat(writeInto(stream)).containsExactly(expected.toByteArray());
+        }
+        if (i % 11 == 0) {
+          assertThat(writeTo(stream)).containsExactly(expected.toByteArray());
+        }
+      }
+      assertThat(writeTo(stream)).containsExactly(expected.toByteArray());
+      assertThat(writeInto(stream)).containsExactly(expected.toByteArray());
+      assertThat(BytesInput.from(stream).toByteArray()).containsExactly(expected.toByteArray());
+    }
+  }
+
+  /**
+   * Reproduces the state of the corrupt pages of GH-3712: a full slab holds fewer bytes than were written to it,
+   * while size() is correct. The content must not be serialized, because it doesn't match the length prefixes and
+   * the page header computed from size().
+   */
+  @ParameterizedTest
+  @MethodSource("allocators")
+  public void testSerializingFailsIfTheContentDoesNotMatchTheSize(ByteBufferAllocator baseAllocator)
+      throws IOException {
+    List<ByteBuffer> slabs = new ArrayList<>();
+    ByteBufferAllocator recordingAllocator = new ByteBufferAllocator() {
+      @Override
+      public ByteBuffer allocate(int size) {
+        ByteBuffer slab = baseAllocator.allocate(size);
+        slabs.add(slab);
+        return slab;
+      }
+
+      @Override
+      public void release(ByteBuffer b) {
+        baseAllocator.release(b);
+      }
+
+      @Override
+      public boolean isDirect() {
+        return baseAllocator.isDirect();
+      }
+    };
+    try (CapacityByteArrayOutputStream stream = newStream(recordingAllocator, 16)) {
+      byte[] data = new byte[10];
+      for (int i = 0; i < 10; i++) {
+        stream.write(data, 0, data.length);
+      }
+      assertThat(slabs).hasSizeGreaterThan(2);
+      ByteBuffer fullSlab = slabs.get(1);
+      assertThat(fullSlab.hasRemaining()).isFalse();
+      fullSlab.position(fullSlab.position() - 2);
+
+      ByteArrayOutputStream out = new ByteArrayOutputStream();
+      assertThatThrownBy(() -> stream.writeTo(out))
+          .isInstanceOf(IllegalStateException.class)
+          .hasMessageStartingWith("The slabs of this stream hold 98 bytes, but 100 bytes were written to it");
+      assertThat(out.size()).isZero();
+      assertThatThrownBy(() -> stream.writeInto(ByteBuffer.allocate(100)))
+          .isInstanceOf(IllegalStateException.class)
+          .hasMessageStartingWith("The slabs of this stream hold 98 bytes, but 100 bytes were written to it");
+      assertThatThrownBy(() -> BytesInput.from(stream).toByteArray())
+          .isInstanceOf(IllegalStateException.class)
+          .hasMessageStartingWith("The slabs of this stream hold 98 bytes, but 100 bytes were written to it");
+    }
+  }
+}