You sure it didn't say "text art" or something to that effect?
Because ASCII is simply a character encoding. If you're typing with the latin alphabet and its extensions, numbers, or most punctuation symbols, you're probably using ASCII.
I can't make it appear anymore, sadly. (As someone probably already linked, this video still has the old comment system:
http://www.youtube.com/watch?v=ljCj01WIZ-Y) I guess sometime between when that happened and the new comment system, they fixed periods causing it. I even tried an ascii generator, but all it said was a general error. They made the statements more general so abusers have a slightly harder time determining what to change about their abusive comments.
Edit: But basically, you'd get an error message with something about ASCII. I don't remember if it spoke about images or not, but I'm 100% sure it was about ASCII.