How do you test your ROMs?

For one, ZSNES doesn't take pixels away from a monster sprite that is too large, or rather, there is normally a certain amount of pixels allowed per scan line or some shit like that and say you use a spell that exceeds it, it will usually take away some pixels from a monster sprite to compensate, ZSNES magically avoids it. This is most noticeable with very large monster sprites or multiple monsters on screen at once sharing the same lines. The original game tried it's best to avoid it from happening much and they did a good job, but you can see it happen now and then.

Another thing is that it allows for exceeding the instrument size limit, meaning you can have more than 3700 blocks of instrument RAM per song without it becoming all glitchy/broken. Again, due to it's inaccuracies it manages to avoid this blaring issue.

Besides those two things, it's just a known thing that it has inaccurate audio and timing and basically everything compared to the actual hardware so it's wise not to use it for testing.