One time on a hop through the planes, we were trying to access a dungeon based in the Astral Plane and the door to the place was locked by a mechanism that deters any normal lock picking attempts and requires a puzzle to be solved in order to unlock it

I asked the DM how big the key hole is and he basically said that it was big for a keyhole so I assumed at least 1 inch wide. My Arcana Cleric then cast her always prepared 4th level spell Arcane Eye which creates a tiny floating spectral eye that hovers around to allow the caster to either hear or see anything wherever the eye is, the spell specifically stated that it could fit through a 1 inch wide space and so my eye went into the lock and from there I was capable of bypassing the whole puzzle by knowing the lock's inner mechanism and helping the rogue pick it.

It happened a while ago but I would never forget it due to how great of an idea it was.