I was recently told one that I enjoyed (this is getting pretty distant, but still).
My neighbour had been a pretty reasonable flanker and captain of the 1sts at Hurlstone Agricultural, after which he'd headed to uni, played for a while, then shipped off to London for his OE.
By the time he got back he was a bit out of shape and a bit out of touch with how Hurlstone was going, but he was still expected to organise and captain the Old Boys in their annual game against the school. When he showed up on game day he gave himself the plum position of outside centre, because he saw a kind of chubby unimpressive kid wearing 13 for the school, and he felt like an easy day.
First time the chubby kid got the ball he steam-rolled old mate and put him out cold.
Turns out it was David Lyons, fresh from aussie schools. My neighbour had never heard of him, but has since painfully learned to respect the presence of muscle under fat.