I'm a gamer who uses the 4th and 5th buttons for certain functions in-game, but sometimes Litestep randomly makes these buttons switch my game to a separate desktop. I'd say this happens roughly half the time I load up a game. Half the time the buttons will perform their in-game function and half the time they'll switch the game to the next desktop. Of course I can easily switch it back with the other button so it's not really a huge problem (mouse4 goes left and 5 goes right) but it is a bit annoying.

Has anyone else come across a problem like this and know how to fix it?

i have the mx500 too, but haven't come across this problem ... maybe it's because i didn't play much the past months.
can't really help you on this so ... senseless post somehow :/

sounds like you have the buttons setup to do that in LS. edit your personal.rc and undo that.