After the escape from Azkaban, I thought that the most likely thing for the Remembrall to have been referring to, must have been the Newtonian physics that Harry was mentioned to have forgotten about in regards to broomstick flying -- but am not quite sure that makes sense anymore. At the time the Remembrall lit up, Harry hadn't spent any time steering/accelerating the broomstick, and had barely seen anyone else fly at all.