Comment 11 for bug 285056

I found the problem it has to do with the reverse mapping in xmodmap. So keycodes are just badly mapped it remove to bad mapping fixes the problem with the program xev you can figure out what the bad keycode is.

for instance:

KeyPress event, serial 31, synthetic NO, window 0x4600001,
    root 0x13b, subw 0x0, time 42893225, (360,-81), root:(365,651),
    state 0x10, keycode 172 (keysym 0x1008ff14, XF86AudioPlay), same_screen YES,
    XKeysymToKeycode returns keycode: 162
    XLookupString gives 0 bytes:
    XmbLookupString gives 0 bytes:
    XFilterEvent returns: False

This one maps back to keycode 162 is stead of 172 so removing 162 from your xmodmap fixes the problem