I saw this big bumbler working a patch of Comfrey. It seemed so large and laden with heaviness that its wings were barely equal to the task of taking flight. And too large to stick its snout into the flowers of the Comfrey.

 Instead it was "nectar robbing". In the photograph below you can just about make out the bee's tongue piercing the base of the flower to get at the nectar. In doing so the robber gets its reward but bypasses the anthers and thereby pollination does not take place.