One of us is a spy

During a goofy short-run Mage campaign (High School Mage: The College Years), while the PCs were preparing to enter the final dungeon, one of the (somewhat intoxicated) players nearly brought the entire campaign to a premature end by exclaiming apropos of nothing, "Wait! You guys! One of us is a spy!" I can't fully explain why I found this so funny but it might still be the hardest I've ever laughed during a gaming session. It is still often brought up when someone (player or GM) is threatening to introduce an eleventh-hour plot complication out of left field.

This meme became suddenly factual when Nomi Amersu's secret past as a Republic spy was discovered in Doomsayer.