[Cassandra]

This one happened some ten years ago.

The characters were trying to find some bandits, who were hiding on a small island. A bridge led to this island, and it was guarded by a few bandits. The PC's fought and won the bandits, and contienued towards the small village on the island, where the rest of the bad guys were supposed to be hiding. When the fight broke out, we found out how much one player had been paying attention.

PC: I find a good spot so I can fire arrows on the bridge.
GM: Uh, what now?
PC: The bandits are on the bridge, right? I want to fire at them.
GM: Um, you're on the island now. You already crossed the bridge.

The last phrase is still uttered at almost every other session, whenever someone is paying less than complete attention.