Usually they wrote us a check for the wrong amount at some point and end up with a small balance remaining in their account. We don't want to just forgive and forget every time somebody owes us $0.16, but if it's been there months and it's going to take extra effort to get it from them, it's probably not worth it.

We do try to determine where the discrepancy started and watch for indicators of a bigger issue.

Or a compromised system https://en.wikipedia.org/wiki/The_Cuckoo%27s_Egg_(book)