Gittin 53 - Mixing kosher and non-kosher foods
If one mixes non-kosher food into the kosher food of his friend - it is a damage that is not visible. He has to pay - if he did it on purpose, but not if by mistake.
Why? In truth, he should NOT be liable in both cases. But if he did it by mistake, he will not tell his friend if he has to pay, so the payment was canceled. If he did it on purpose, he will announce it anyway.
Or, in truth he SHOULD be liable in both cases. But they imposed a penalty against intentional damages.