Sprite zero hit
The NES flag that trips the instant the first sprite's pixels touch the background — a game's only clock in the middle of a frame.
The PPU had no interrupts tied to individual scanlines. The workaround was a side effect: when an opaque pixel of sprite 0 overlapped an opaque background pixel, a flag was set in the status register ($2002). By parking sprite 0 at the boundary between playfield and status bar and spinning in a tight loop until the flag tripped, code could reload the scroll register on exactly the right line. Super Mario Bros. (1985) held its score bar still this way. The trick was brittle — a dropped cycle glitched the picture — and gave way on beefier cartridges to the mapper's own scanline counter.