Hands down, the most popular question on the paper money forum, "What's this random mark on my note and is it an error ?" I see at least one post a week asking this. I'm not aware of a website that lists or illustrates random, stray bank stamps on paper money. If you're familiar with the printing process, you're probably more likely to figure it out.
Banks and other financial institutions get packs of currency banded with a paper strap. A teller is tasked with stamping the band with an inked stamp, typically the bank name and maybe a date. Subsequently, some of the tellers aim is better than the others and the stamp hits both the strap and part of the note leaving an ink mark visible when the strap is removed.