"Rare" versus "common" is an interesting one. They sound like antonyms, but I don't think the typical probabilities are really symmetrical. Maybe something like 0%-10% for rare (although some sources say 5%) and something like 40%-100% for common.
if you're a teacher and one student per class does the same thing - it's common. Even though it's only 1/25 or 1/30 of all students
What you're describing, I think I would call "Not uncommon." Or, to put it another way, you shouldn't be surprised for any given case to exhibit it, but you shouldn't expect it either.