I've had this happen even with Hitec servo arms, specifically the long black ones that come with the servos. It just happened on a 3D plank I have yesterday. the best fix is to go with the thick, red X type servo arms from Hitec. They never seem to slip.

Normally this will occur if you use Futaba arms on a Hitec servo. It'll work at first, but after the servo has been under a load for a while it will slip.