While rowing, "catching a crab" means to put one's oar in the water at the wrong time which results in the oar flipping parallel to the boat. When an oar goes from perpindicular (good) to parallel (bad) a rower must pull the oar in and over their head.
This can be difficult to fix as the boat moves.
Kate: I was rowing Port seat 6 yesterday when I totally caught a crab. It was horrible.
Ben: I remember! I was in 5 seat so I stopped to help you get your oar in order.
Kate: Thanks. It was intense. I hope I don't catch a crab at next Saturday's regatta!