Exactly. Why not make them crit? It's going to be up to the DM anyway to define what a "critical success" means on a skill check. There's no hard rule like the extra damage that comes with crit successes on attacks. The DM gets to choose what a critical success on a skill check actually produces. The DM can easily just make sure the crit success isn't game breaking.

Your players are in an audience with the king. The bard tries to be funny and tries to convince the king to give him his crown and hand the kingdom over to him. Actually making the bard the new king would break the game. But maybe a critical fail means the bard gets sent to the dungeon to be tortured for daring to make such a request. A critical success means the king will grant the bard one "wish," ie, any reasonable single reasonable request that is within the king's power.

The whole situation is fully in the DM's power.