Craft PNG files that appear completely different in Apple software
For context: https://www.da.vidbuchanan.co.uk/widgets/pngdiff/
If you're viewing this via Apple software (e.g. Safari) you should see an image of a mac, and on other non-Apple software, you should see an IBM PC. (Note: If you're in Safari and still seeing the IBM PC, hit refresh a few times...)
As a bonus, here's a race condition I found in desktop macOS Safari:
You should see a slightly different image on each page refresh!
Here's a "live" version, which self-refreshes on each frame: https://www.da.vidbuchanan.co.uk/widgets/pngdiff/race.html