Far from the worst but my funniest example comes from my favourite show. In the first season of Stargate SG-1, they introduced an alien weapon which would go on to become a staple of the series, the Zat Gun. One shot stuns, two shots kill, and three shots... Disintegrates?!
Yeah, it's so stupidly powerful that the writers pretty much immediately realized the mistake and the show kinda just conveniently forgets that was established in the first season, except for like three other times in the entire ten season run.
Even the first two shots became very nebulous over time.
Edited to add, the disintegration effect is even mocked in-world in a very meta way in later seasons.