Originally Posted by Shimagnolo

In my experience, a chain can be long enough to handle the big/big combo, but still be too short to function properly. In any gear in which there is any cross-chaining whatsoever, as the lower pulley is pulled forward of the rear axle, it moves out of the plane in which the chain is running. I had a chain attempting to derail off the side of the pulley in this situation, and had to lengthen the chain to allow the pulley to move toward the back. This was on a 9s Shimano 9s with 22/32/44 and 11-36. A chain too short is a much more serious problem than having some slack in the small/small.