PS1 had an interlaced mode where IIRC you had to keep up with 50/60Hz, because there were only two screen pages, and every other vertical retrace would use one

PS1 had an interlaced mode where IIRC you had to keep up with 50/60Hz, because there were only two screen pages, and every other vertical retrace would use one of them. You're rendering to buffer A while B shows, but if you're not done with A at the vertical retrace, A will start showing while you are still rendering to it. That's problem 1. Problem 2 is, can you render the next frame to B in the less-than-a-frame time you have left before B shows again? If you discard the the current partial frame and wait, the old B will show, effectively looking like a backwards movement glitch. You would be right in that few games dared use it for gameplay. Gran Turismo (Japan import only, maybe?) had a hi res mode where it used this interlaced mode, and had to simplify the environment a lot compare to the regular game. Watching that thing run at 60Hz in high resolution was breathtaking.