When someone you're talking to over AIM types something for a while, then backspaces it and doesn't send it, and you only knew they were about to say something because your IM client shows when they're typing something?
The backspaced IM is a "phantom im". It often really is something better left unsaid.
Sometimes confused with "typing something meant for someone else in the wrong window".
"I asked Mike what his phantom im was, but it was something I wish I hadn't asked about."