Undo information for sprite/map data is kept across "reboots" (using 0.1.4b under Linux):

• Start a fresh copy of Pico-8
• Open sprite editor and draw anything
• Press UNDO
• Drop to the console, reboot pico 8
• Re-open sprite editor and press Undo

I would expect this to do nothing, but in fact what you drew appears back in the sprites. It also sometimes pulls back several sprites and not just the most recent thing that you undid, although I have not determined any pattern to this.

This doesn't affect code, sound or music, just sprite and map data; the map data is subtle because after a reboot the sprites are empty but pressing undo there will insert the previous tile data, which then visually has no change.

Version 0.1.3 does not exhibit this problem as far as I have been able to reproduce; there it works as expected.