I had a client once who refused to pay for one particular day of working. They paid everything else, but not that particular day, which was odd. (It may have been a day I worked from home and didn't push any commits, so maybe that was the reason.)

I think it was about € 600. I spoke to a lawyer, may have written a letter threatening a lawsuit even, but the lawyer explained that it just wasn't enough to actually start a lawsuit over. Even if you win, lawsuits take a lot of time and energy. I just dropped it. And them, obviously.

A different case from the opposite side:

I'm currently looking at a disagreement with my previous phone provider. They charged me too much over the past year. I called them about it, and they wanted me to prove we agreed on the monthly fees I claimed, and not the ones they claim. But they can't claim we agreed on their fees, so I just stopped paying and switched to a different (better and cheaper) phone provider (which I should have done a long time ago anyway). They're threatening to cut my phone which already isn't with them. They want money from me, but yesterday I wrote them a letter explaining they actually owe me money.

I expect this will go away. They may register me somewhere as a bad-payer, but there's an appeal process that I expect will side with me, because the phone company can't prove a thing.