Every time X11 is upgraded, drivers break. Often, the graphics card driver will still work, but my keyboard and mouse (evdev) always stop working, and it takes some head-scratching for me to remember what to do. The solution is to re-emerge the drivers.
But this is too fragile and should be unnecessary.
Either the drivers should be automatically re-emerged just to be safe, or version checks need to be made by portage to determine when driver binaries are out of date relative to X11 and need to be recompiled.
Fixing this would eliminate the flood of crying for help on IRC every time this happens, which is a LOT.
*** This bug has been marked as a duplicate of bug 192319 ***