Sprite flicker
The telltale shimmer when more sprites crowd onto one scanline than the hardware can draw, and the engine lets them take turns vanishing.
The NES PPU drew at most eight sprites per scanline; the ninth and beyond were dropped. Rather than leave the same object invisible every frame, games rotated which sprite got lowest priority, so the dropout wandered — the eye read flicker instead of a permanent hole. Shoot-'em-ups and bosses spraying projectiles flickered worst. It was not a bug but a deliberate choice between two bad outcomes. The C64 dodged the problem in part with sprite multiplexing; later consoles simply raised the limit.