I read that while tipping in Poland is standard, you should not say "thank you" to the waiter when he takes the cash if you are expecting change. Otherwise, he will assume that the amount overpaid is his tip. Thus, if you want to leave less of a tip than what you gave, you should hold all "thank you"s until after he has returned with the change.
Is this true?
This seems to match with my experience (one data point). We only had large bills to pay for dinner, we politely said "thank you" when the waiter took the tab, and we never received change. We couldn't find the waiter afterwards and the amount we over-tipped wasn't too much, so we just let it go.