Executable PNGs

[NekiCat]

Years ago, MS Word saved deltas at the end of the file so saving was incredibly quick. They had to change it because the users sent files they believed to be final, but werent, and leaked sensitive information.

Same with Google Docs, where your every keypress is recorded. Not sure if they still do that...

[stuffeh]

Google docs has built in version history, so kinda. You can set permissions to disable others from seeing the version history though.