diff -r 3ae2cd85a908 Lib/gzip.py
--- a/Lib/gzip.py	Sun Mar 09 11:18:16 2014 +0100
+++ b/Lib/gzip.py	Sun Mar 09 21:41:08 2014 +0200
@@ -96,7 +96,7 @@
             self._read -= len(prepend)
             return
         else:
-            self._buffer = self._buffer[read:] + prepend
+            self._buffer = self._buffer[self._read:] + prepend
         self._length = len(self._buffer)
         self._read = 0
 
diff -r 3ae2cd85a908 Lib/test/test_gzip.py
--- a/Lib/test/test_gzip.py	Sun Mar 09 11:18:16 2014 +0100
+++ b/Lib/test/test_gzip.py	Sun Mar 09 21:41:08 2014 +0200
@@ -421,6 +421,13 @@
         with gzip.GzipFile(fileobj=io.BytesIO(gzdata)) as f:
             self.assertEqual(f.read(), b'Test')
 
+    def test_prepend_error(self):
+        # See issue #20875
+        with gzip.open(self.filename, "wb") as f:
+            f.write(data1)
+        with gzip.open(self.filename, "rb") as f:
+            f.fileobj.prepend()
+
 class TestOpen(BaseTest):
     def test_binary_modes(self):
         uncompressed = data1 * 50