So there was this gargantuan shark. This was one of the first games I ran, so I didn't have a good sense for CRs, and this creature was much too powerful for the party. It could have 1-shot any of the PCs if it got a solid attack in.

Luckily, attempting to bite a PC, it rolled a 1. Then it rolled a 1 to confirm its critical fumble.

In short, the gargantuan shark chewed itself to death in its frenzy to eat the PCs.