diff --git a/Lib/test/test_io.py b/Lib/test/test_io.py
index d32580c..72c9a2d 100644
--- a/Lib/test/test_io.py
+++ b/Lib/test/test_io.py
@@ -1395,15 +1395,18 @@
         rw.seek(0, 0)
         self.assertEqual(b"asdf", rw.read(4))
 
-        rw.write(b"asdf")
+        rw.write(b"123f")
         rw.seek(0, 0)
-        self.assertEqual(b"asdfasdfl", rw.read())
+        self.assertEqual(b"asdf123fl", rw.read())
         self.assertEqual(9, rw.tell())
         rw.seek(-4, 2)
         self.assertEqual(5, rw.tell())
         rw.seek(2, 1)
         self.assertEqual(7, rw.tell())
         self.assertEqual(b"fl", rw.read(11))
+        rw.flush()
+        self.assertEqual(b"asdf123fl", raw.getvalue())
+
         self.assertRaises(TypeError, rw.seek, 0.0)
 
     def check_flush_and_read(self, read_func):
@@ -1548,6 +1551,43 @@
         BufferedReaderTest.test_misbehaved_io(self)
         BufferedWriterTest.test_misbehaved_io(self)
 
+    def test_interleaved_read_write(self):
+        # Test for issue #12213
+        with self.BytesIO(b'abcdefgh') as raw:
+            with self.tp(raw, 100) as f:
+                f.write(b"1")
+                self.assertEqual(f.read(1), b'b')
+                f.write(b'2')
+                self.assertEqual(f.read1(1), b'd')
+                f.write(b'3')
+                buf = bytearray(1)
+                f.readinto(buf)
+                self.assertEqual(buf, b'f')
+                f.write(b'4')
+                self.assertEqual(f.peek(1), b'h')
+                f.flush()
+                self.assertEqual(raw.getvalue(), b'1b2d3f4h')
+
+        with self.BytesIO(b'abc') as raw:
+            with self.tp(raw, 100) as f:
+                self.assertEqual(f.read(1), b'a')
+                f.write(b"2")
+                self.assertEqual(f.read(1), b'c')
+                f.flush()
+                self.assertEqual(raw.getvalue(), b'a2c')
+
+    def test_interleaved_readline_write(self):
+        with self.BytesIO(b'ab\ncdef\ng\n') as raw:
+            with self.tp(raw) as f:
+                f.write(b'1')
+                self.assertEqual(f.readline(), b'b\n')
+                f.write(b'2')
+                self.assertEqual(f.readline(), b'def\n')
+                f.write(b'3')
+                self.assertEqual(f.readline(), b'\n')
+                f.flush()
+                self.assertEqual(raw.getvalue(), b'1b\n2def\n3\n')
+
     # You can't construct a BufferedRandom over a non-seekable stream.
     test_unseekable = None