Low Donations

If you're, for example, a streamer and you get multiple very low value donations from a new viewer, you should just refund all of them. If you don't, that viewer can request a Paypal charge back on all of them. Depending on certain scenarios, each charge back will cost you 8 to 20$, potentially even higher. That's regardless of the amount being charged back.

Yes, that's illegal, but you can't really prevent it.