If you got something like this http://cs2bus.com/sharedfiles/filedetails/?id=215402667&tscn=1389463100, check this out.

Fix by LuxioTheGamer

1.- Disable Steam Community for Quake III (Right Click on Quake III Arena in your library, then properties and finally uncheck the box that says something about enabling the steam community in-game. Disable the box)
2.- Enter to Quake 3 and next go to Setup, disable the option GL Extensions
3.- Quit, next go to the library and again, enable the Steam Community
4.- Rock Hard in Quake!

Un Fix por LuxioTheGamer
Translated in spanish by me, si tienen preguntas acerca del error, pueden preguntarme en mi perfil.

1.- Deshabilita Steam Community para Quake III, ve a la libreria, click derecho sobre Q3, ve a propiedades y finalmente deshabilita el cuadrito que dice Steam Community for this game, o algo asi :D
2.- Entra al Quake 3, despues ve a la opcion Setup, ahi deshabilitas la opcion de GL Extensions o Extensiones de GL
3.- Quita el juego, ve a la libreria y habilita la opcion de La Comunidad de Steam
4.- Dale Duro al Quake 3!!!