For what it's worth, here's an example of two PNGs which differ by 1 bit, in the length. http://dalkescientific.com/xo.png http://dalkescientific.com/xo.png is

For what it's worth, here's an example of two PNGs which differ by 1 bit, in the length. http://dalkescientific.com/xo.png http://dalkescientific.com/xo.png is a PNG for an "O" with two IDAT blocks: 'IHDR' 13 (this is the chunk data size, excluding the 4 bytes of crc) 'xtra' 0 'IDAT' 1012 <-- this one is displayed 'IDAT' 1090 <-- this one is ignored 'IEND' 0 http://dalkescientific.com/xo_bad.png http://dalkescientific.com/xo_bad.png is an invalid PNG for an "X" with one IDAT block and with an invalid checksum: 'IHDR' 13 'xtra' 1024 <-- the extra length contains what was the first IDAT 'IDAT' 1090 <-- this one is displayed 'IEND' 0 Both display in Preview.app, Firefox, and Safari. They differ only in byte offset 35, where one has a chr(0) and the other a chr(4): % python -c 'print(set(enumerate(open("xo.png", "rb").read()))\ .symmetric_difference(\ set(enumerate(open("xo_bad.png", "rb").read()))))' {(35, 0), (35, 4)} With a bit more work I could probably construct PNGs which are both valid, and which differ only by a bit. It would require brute-forcing some chunk data so the CRCs would get a match. I have some ideas on how to make it so there's only one valid (and different) IDAT block as well, but that's more than I can do in an evening.