This is the best one I've ever seen. The description seems fairly benign, but then one needs a ridiculous number like A(187196) to describe the applications of

This is the best one I've ever seen. The description seems fairly benign, but then one needs a ridiculous number like A(187196) to describe the applications of Ackermann's function to get a weak lower bound. And the first two steps are rather small, which adds to the surprise.