Above was in 64-bit mode (x86_64).  Same run in 32-bit (i386):

$ ./bin/python3.5-32 /tmp/b/bytearray_bug.py
buf2: 13 2 bytearray(b'1234567890123')
buf1: 13 11 bytearray(b'1234567890123')
buf2: 24 16 bytearray(b'345678901231234567890123')
buf1: 15 4 bytearray(b'231234567890123')
buf2: 21 16 bytearray(b'678901231234567890123')
buf1: 24 3 bytearray(b'345678901231234567890123')
buf2: 18 1 bytearray(b'901231234567890123')
buf1: 34 28 bytearray(b'6789012312345678901231234567890123')
buf2: 30 21 bytearray(b'012312345678901231234567890123')
buf1: 19 8 bytearray(b'8901231234567890123')
buf2: 22 6 bytearray(b'5678901231234567890123')
buf1: 24 11 bytearray(b'345678901231234567890123')
buf2: 29 23 bytearray(b'12312345678901231234567890123')
buf1: 26 15 bytearray(b'12345678901231234567890123')
buf2: 19 15 bytearray(b'8901231234567890123')
buf1: 24 4 bytearray(b'345678901231234567890123')
buf2: 17 7 bytearray(b'01231234567890123')
buf1: 33 21 bytearray(b'789012312345678901231234567890123')
buf2: 23 13 bytearray(b'45678901231234567890123')
buf1: 25 20 bytearray(b'2345678901231234567890123')
buf2: 23 8 bytearray(b'45678901231234567890123')
buf1: 18 7 bytearray(b'901231234567890123')
buf2: 28 3 bytearray(b'2312345678901231234567890123')
buf1: 24 8 bytearray(b'345678901231234567890123')
buf2: 38 1 bytearray(b'23456789012312345678901231234567890123')
buf1: 29 7 bytearray(b'12312345678901231234567890123')
buf2: 50 38 bytearray(b'34567890123123456789012312345678901231234567890123')
buf1: 35 7 bytearray(b'56789012312345678901231234567890123')
buf2: 25 4 bytearray(b'2345678901231234567890123')
buf1: 41 20 bytearray(b'23123456789012312345678901231234567890123')
buf2: 34 24 bytearray(b'6789012312345678901231234567890123')
buf1: 34 19 bytearray(b'6789012312345678901231234567890123')
buf2: 23 1 bytearray(b'45678901231234567890123')
buf1: 28 18 bytearray(b'2312345678901231234567890123')
buf2: 35 20 bytearray(b'56789012312345678901231234567890123')
buf1: 23 13 bytearray(b'45678901231234567890123')
buf2: 28 16 bytearray(b'2312345678901231234567890123')
buf1: 23 19 bytearray(b'45678901231234567890123')
buf2: 25 16 bytearray(b'2345678901231234567890123')
buf1: 17 4 bytearray(b'01231234567890123')
buf2: 22 18 bytearray(b'5678901231234567890123')
buf1: 26 18 bytearray(b'12345678901231234567890123')
buf2: 17 14 bytearray(b'01231234567890123')
buf1: 21 18 bytearray(b'678901231234567890123')
buf2: 16 14 bytearray(b'1231234567890123')
buf1: 16 11 bytearray(b'1231234567890123')
buf2: 15 10 bytearray(b'231234567890123')
buf1: 18 2 bytearray(b'901231234567890123')
buf2: 18 2 bytearray(b'901231234567890123')
buf1: 29 3 bytearray(b'12312345678901231234567890123')
buf2: 29 11 bytearray(b'12312345678901231234567890123')
buf1: 39 9 bytearray(b'123456789012312345678901231234567890123')
buf2: 31 23 bytearray(b'9012312345678901231234567890123')
buf1: 43 31 bytearray(b'0123123456789012312345678901231234567890123')
Debug memory block at address p=0x7aec88: API 'o'
    49 bytes originally requested
    The 3 pad bytes at p-3 are FORBIDDENBYTE, as expected.
    The 4 pad bytes at tail=0x7aecb9 are not all FORBIDDENBYTE (0xfb):
        at tail+0: 0x31 *** OUCH
        at tail+1: 0x32 *** OUCH
        at tail+2: 0x33 *** OUCH
        at tail+3: 0x00 *** OUCH
    The block was made by call #44809 to debug malloc/realloc.
    Data at p: 31 32 33 34 35 36 37 38 ... 33 34 35 36 37 38 39 30
Fatal Python error: bad trailing pad byte

Current thread 0xa0ddc1d4 (most recent call first):
  File "/tmp/b/bytearray_bug.py", line 25 in <module>
Abort trap: 6