Your college roommate, the one who "accidentally" took all your food from the fridge and "forgot" to do their laundry for three months straight. You thought you were friends, but deep down you knew you were just enabling their slobbery ways.
But then there's the friend from that one party in 2007 who "totally" lost their phone number and now can't remember your name. That's a whole different story.
Or maybe you're just thinking about that one acquaintance who still owes you 5 bucks from 2012. That's a real mystery.