The producer or the director (I forget which) pointed this gaffe out himself, on the commentary track of the CAV laserdisc: In the film's climax, Simba and Scar battle to the death while Pride Rock burns around them. However, under Scar's rule, Pride Rock had already been reduced to a barren wasteland. So, there are no trees. There's no grass. There aren't even two dry sticks to rub together. In other words, THERE'S NOTHING LEFT TO BURN!!

I'd also like to point out something I noticed myself: Go to the very beginning of Timon's "hula" scene (before he yells "Luau!") and watch the first shot, where Timon's back is turned, in slow motion. You can clearly see that he's not wearing a lei. Keep watching this shot in slo-mo. Just before he turns around, the lei will suddenly appear around his neck.