The excellent bash shell keeps a history of recently executed command lines and lets users cycle through them with up/down arrow, edit commands, and re-execute past commands.

Every once in a while, I make a mistake and type a password at a bash prompt -- today it happened because I had a remote session waiting for a password, which timed out, so I typed my password at the "Password:" prompt, but my workstation noticed the connection was down and dropped me into bash, which failed to execute my password (because it's of course not a valid command), and helpfully cached it on disk for future shells to take advantage of.

The quick fix is "history -c", to flush the whole history (and Command-K in Terminal or whatever's necessary in a different terminal program to clear the screen and scroll-back buffer).

A less drastic step is to use "history | tail" to find the line number of the bad command, and "history -d 503" (or whatever the appropriate line number is) to clear just the bad line, preserving the rest of the history. Further details are available with "man bash".