Reproduced with top of trunk default branch on OS X (so, not Windows-specific).  With debug enabled:

$ ./bin/python3.5 /tmp/b/bytearray_bug.py
buf2: 13 5 bytearray(b'1234567890123')
buf1: 13 3 bytearray(b'1234567890123')
buf2: 21 2 bytearray(b'678901231234567890123')
buf1: 23 5 bytearray(b'45678901231234567890123')
buf2: 32 28 bytearray(b'89012312345678901231234567890123')
buf1: 31 21 bytearray(b'9012312345678901231234567890123')
buf2: 17 10 bytearray(b'01231234567890123')
buf1: 23 9 bytearray(b'45678901231234567890123')
buf2: 20 3 bytearray(b'78901231234567890123')
buf1: 27 14 bytearray(b'312345678901231234567890123')
buf2: 30 22 bytearray(b'012312345678901231234567890123')
buf1: 26 17 bytearray(b'12345678901231234567890123')
buf2: 21 1 bytearray(b'678901231234567890123')
buf1: 22 12 bytearray(b'5678901231234567890123')
buf2: 33 13 bytearray(b'789012312345678901231234567890123')
buf1: 23 4 bytearray(b'45678901231234567890123')
buf2: 33 23 bytearray(b'789012312345678901231234567890123')
buf1: 32 4 bytearray(b'89012312345678901231234567890123')
buf2: 23 19 bytearray(b'45678901231234567890123')
buf1: 41 36 bytearray(b'23123456789012312345678901231234567890123')
buf2: 17 3 bytearray(b'01231234567890123')
buf1: 18 3 bytearray(b'901231234567890123')
buf2: 27 23 bytearray(b'312345678901231234567890123')
buf1: 28 22 bytearray(b'2312345678901231234567890123')
buf2: 17 2 bytearray(b'01231234567890123')
buf1: 19 5 bytearray(b'8901231234567890123')
buf2: 28 10 bytearray(b'2312345678901231234567890123')
buf1: 27 2 bytearray(b'312345678901231234567890123')
buf2: 31 7 bytearray(b'9012312345678901231234567890123')
buf1: 38 2 bytearray(b'23456789012312345678901231234567890123')
buf2: 37 23 bytearray(b'3456789012312345678901231234567890123')
buf1: 49 2 bytearray(b'4567890123123456789012312345678901231234567890123')
buf2: 27 21 bytearray(b'312345678901231234567890123')
buf1: 60 1 bytearray(b'678901231234567890123123456789012312345678901231234567890123')
Debug memory block at address p=0x10bd238e0: API 'o'
    61 bytes originally requested
    The 7 pad bytes at p-7 are FORBIDDENBYTE, as expected.
    The 8 pad bytes at tail=0x10bd2391d are not all FORBIDDENBYTE (0xfb):
        at tail+0: 0x33 *** OUCH
        at tail+1: 0x00 *** OUCH
        at tail+2: 0xfb
        at tail+3: 0xfb
        at tail+4: 0xfb
        at tail+5: 0xfb
        at tail+6: 0xfb
        at tail+7: 0xfb
    The block was made by call #44842 to debug malloc/realloc.
    Data at p: 34 35 36 37 38 39 30 31 ... 35 36 37 38 39 30 31 32
Fatal Python error: bad trailing pad byte

Current thread 0x00007fff70e18300 (most recent call first):
  File "/tmp/b/bytearray_bug.py", line 25 in <module>
Abort trap: 6