So I hit command-A, command-O (select all, open) only to discover a millisecond too late that I wasn't in the window I thought I was and had accidentally opened over 500 diverse documents in a dozen-odd apps between them all at once.
Obviously not the end of the world, but definitely annoying.
Way back when Macs were actually Macs instead of PC clones running a Unix-based OS, typing command-period immediately after making such a dumb mistake would nullify said mistake and make everything right again by canceling the app openings before they started fighting over RAM and clock cycles. It was a very useful feature that fixed a lot of boneheaded mistakes in the past.
Unfortunately, that feature is no longer in place. And it needs to come back.