I recently had an issue where my keyboard when using synergy was inserting a double arrow instead of the usual greater than or less than sign also known as guillemotleft and guillemotright.
To fix this annoying bug I added
keycode 53 = x X greater greater greater greater
keycode 52 = z Z less less less less
keycode 24 = q Q at at at at
to a .Xmodmap in my home directory. If you are wondering how to find the keycodes etc you can use xev. To start it without restarting X use