I've noticed some weird graphical artifacts with NES emulation (that happen with any and all emulators I tried - Nestopia, FCEUltra, VisualNES to name a few).
This specifix issue I noticed with Super Mario Bros 3, but I saw it happen in other games with scrolling game screens. Basically, when you move to the right or the left, so that the screen scrolls, you get narrow column bars on both sides of the screen that display wrong colors or completely wrong sprites.
In SMB3, for example, when you walk to the right (very first stage) the right side of the screen has a vertical bar several pixels wide that displays bad colors for the sky, pipes, and blocks as they scroll into view.
I don't remember this artifact being in the game on the NES, but that was a long time ago... or perhaps it gets hidden somehow by the TV?