Well, the fact that he remembered the night of Voldemort’s attack (I have 80%+ confidence it was a real memory, though I’m not sure of what), it’s clear that there was at least one thing that he had forgotten at the time he held the Remembrall. I hadn’t made the connection until now. Previously I thought the crimson eyes were a hint pointing to Moaning Myrtle’s recounting of her encounter with the basilisk. Now I wonder if it’s just a coincidence because the “blazing sun” simile works well for a bright Remembrall, or if Eliezer is giving us a three-way hint. A basilisk being present at Godric’s Hollow had a lot of awesomeness potential, but it does seem like the less likely hypothesis.

On the other hand, pretty much everyone has forgotten most of their infancy—presumably, Dementors don’t bring such memories back because they’re almost never as traumatic as Harry’s—and Remembralls don’t start glowing like mad in everybody’s hands, so something is still missing there.

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.