Team fortress particle (probably) issues
Well, I'm having this problem in TF2, I can't see medic beams or pyro flames (people on flames look like "lava", and I only see the short blue flame and the sparks from the pyro's flame thrower, making it really hard to know how far it reaches).
Once when the game got forced out of fullscreen (an f-secure message or something) and I put it back, they worked for a while, but then after a while (it again jammed and got out of fullscreen) they stopped again.
It's not my computer, I'm pretty sure about that, because the game runs beautifully otherwise, as do all the other games in the orange box. What I did notice is that the flames in HL2: episodes 1 and 2 won't show either, but in HL2 they did.