1. HTML reserved characters like < > and \.
2. overflowing content
3. displaying ASCII made with special encodings and characters sets, like code page 437
4. displaying nonprintable (control) characters
5. displaying colors
6. text preprocessors messing up the formatting
Color is very much a part of ASCII art, if we consider ASCII to mean a broader range of different technologies. Just check the stuff in the article, or the stuff on https://16colo.rs/ site.