I disagree with some of the suggestions so far. I don't think the stinger can be something that the creators thought were funny. It should be something that was unintentionally funny. My suggestions:
I agree with Darth Geek about something from Shy Beef's physics class. Perhaps the first moment when he's looking at his book, and his eyes roll back.
The moment when everyone's all sad about Optimus dying, then they just drop his robot carcass 100 feet from the helicopters.