Lightmap Problem.

[Aqualys]

From my own knowledge and experience, this can happen to absolutely anyone regardless of their video/graphics card or how up to date they are, and a few specific maps are more prone to have this problem (Geffen fields, Byalan B3 and more).

Like someone said before, you can use /lightmap for a temporary fix, or close the client and restart the game for the regular fix (everything comes back to normal). Merely going back to the character selection screen doesn't fix it.