If you've had a darkplaces/epsilon installation at any point prior to v2.5, and you're experiencing any visual problems such as missing textures on weapons or low-quality textures on walls/floors etc, delete the following folder:
Windows XP: My Documents\My Games\darkplaces
Windows Vista/7/8: C:\Users\"USER NAME"\Saved Games\darkplaces (where "USER NAME" is your windows login name)
Linux: ~/.darkplaces
OSX: ~/Library/Application Support/darkplaces
This should fix it but will also delete savegames. For some users, just deleting the config.cfg file within the subfolders of this folder seems to be enough.
On Debian/Lubuntu this doesn't work, there is a issue with those distros and darkplaces.
You may have to re-adjust your screen resolution/FOV/etc settings after you've done this, to match what you had prior to v2.5.
Working on a more permanent fix for it.