I have seen it cause a slow roll on a CRT which is pretty annoying if it happens at the wrong time in a game. Seems to really vary between CRTs. Doesn't happen on newer displays.
The weirdest bug in Halo2600 lets the player enter what I called "Magic Land" and is documented elsewhere online (for example here: http://infinitelives.net/2010/09/16/hello-homebrew-halo-2600/#sthash.1CZlvLrs.dpbs). It's caused by a problem in the hit test code. If the master chief is hitting a wall he gets moved back to the "safe position" where he was in the previous frame. Seems like a good solution except in the case where the player has also crossed onto a different screen on the map between those two frames. In that case the player can be teleported outside the normal 64 rooms and the game code starts constructing rooms and monsters based on random data.