I was made a little suspicious today when reading the lips of some cartoon character and wondered if the fact has ever came out into public that the lips were mouthing profanity instead of the appropriate syncronizations with the soundtrack.
I’m not aware of such a thing and I doubt it can happen. In your typical television animation not enough mouth movement is animated to make a really useful thing for reading lips. Animation with enough detail in the mouth animation so that you can reasonably read the lips is being done to a pre-recorded track and so there just isn’t room for an animator to ad-lib. If you notice any real differences where otherwise it does match reasonably well they’re more likely due to some ADR than crazy animators.