Packer.pack_ext_type() writes to the internal buffer but never checks self._autoreset. Every other public pack method (pack, pack_map_pairs, pack_array_header, pack_map_header) has this pattern:

if self._autoreset:
 ret = self._buffer.getvalue()
 self._buffer = BytesIO()
 return ret

Without it, pack_ext_type() always returns None, and the packed ext data stays in the buffer. The next call to pack() then returns both the extension data and the new value concatenated together, corrupting the serialized stream.

packer = msgpack.Packer()
result = packer.pack_ext_type(5, b'\x01\x02\x03')
# result is None, expected bytes
packer.pack(99)
# Returns 7 bytes (ext header + ext data + int) instead of 1 byte