I'm a pescetarian, and I accidentally took a gum not knowing that there was some beef gelatin in it. Am I allowed to do that?
Gelatin is not vegan, vegetarian nor pescetarian. Gelatin is made of animal collagen.
Collagen is extracted from animals' (pigs' or cows') hides and bones through a chemical process.
I would not worry that much for such an accident. It happens. Being pescetarian/vegetarian/vegan is a choice, an accident does not make you less of those.
It's up to you what you allow yourself to do and how you set your taboos. By definition, gelatin is not vegetarian, as it's made of a slaughtered animal. However, it's only a byproduct and not what the cow was actually killed for, so I can see an argument for being unbothered by it, and I won't take an issue with you eating it and still calling yourself vegetarian (or pescetarian).