https://www.da.vidbuchanan.co.uk/widgets/pngdiff/

PNG Parser Differential

This PNG image renders differently in Apple vs non-Apple decoders:

[a]

If you're using Apple-flavoured software, you should see "HELLO
APPLE", and on all other software, you should see "HELLO WORLD".

I found this while writing my own multi-threaded PNG decoder. While
pondering my design, I realised that I had an exploitable
implementation bug. After learning that Apple has their own
implementation of parallel-decodable PNGs, I realised that they'd
made exactly the same mistake!

Update:

I created a tool to merge arbitrary full-colour images. Here is an
example:

[mac_vs_ibm]