I had several of them fail on flap applications. I sent them back to Horizon, and their instructions upon repair and return were to check to make sure there was no binding of the control surface. My hinges were surface hinges, painted, so they were a bit stiff, particularly at full throw. I concluded that the 2721's did not handle high resistance applications well. Switched to 8611's and never had another issue.

The good news was the lockup failures were preceded by twitching, so I found the problem on the ground, not in the air.