So, say you’ve typed in:
$ sudo rm ruby gem irb rails
Then you realise you’re sitting in /usr/bin rather than ~/test/bin. You think to yourself, no worries, I’ll just kill that whole line. But instead of hitting C-k, you miss, and hit j.
I’ve been using emacs for around 7 years, and until this morning I didn’t know that C-j is the same as hitting enter. 2 hours later, I have a functioning rails install again.