Ever since I switched to the new extra low profile main sheet blocks I've had trouble getting the sheeting to work correctly. The main sheet always wanted to jump the sheave when I was sheeted block to block. Finally at North Americans I figured out what the smart guys were doing and rigged up something temporary. Today I actually moved the becket to make it permanent. This system evidently works great. The trick is moving the becket from on top of the cam cleat to the side of the block.
The two blocks are Harken H2640 and H194.