The Rugrats episode you might be thinking of was one where Chucky was being potty trained. Angelica keeps teasing him about being a baby. He sits on his potty for forever to try to go and can't, and then the second he stands up, of course he pees all over the floor. Eventually he figures it out and uses his potty. At that moment, Angelica starts yelling from her bedroom - she's wet her bed.
Sorry that's not more detailed, but it's been at least 10 years since I've seen that episode.
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