Trying to access memory that isn't there could cause a crash but it also depends on what it's trying to do with it. If it's graphics, then it will more than likely just display garbage.

Here's an example from a game I was working on several years ago. (10 points if you can guess where I borrowed the images for the last 2)


Here the pallets just don't match but it's probably playable. If you were making a game playable on both systems, you would have to pay special attention to the colors are arranged. The black outlines around the characters for example are lighter colored on the GB. I'm also not accessing any of the additional ram so the tiles are all correct...just the wrong colors.



The same goes for my "select" screen. I'm not matching colors between pallets so the backgrounds behind some objects appear as squares.



This was part of the Start screen. Same as before, the colors in the pallets don't match so you see squares. This could be taken into account to look correctly on both, it's just a lot more work.



Here's where things get interesting. In this one, I'm accessing memory that the the original Game Boy doesn't have. The bottom half looks almost OK, but the top appears to be trying to pull from the same tileset instead of what's in the extra ram.


What actually happens would depend completely on the game. Game data is stored on the cartridge in the same way regardless of the system so it would depend on what it does with it in the system ram.

It would pretty easy to program an Easter egg in though but I don't know of any game that has ever done it.