I was looking through my steam profile and I noticed that the Lamb2Slaughter achievement I got didnt register with steam.
After about 30 mins looking through the Valve forums
I found thisYou need to input 3 commands into the console in this order
sv_cheats 1; achievement_reset L4D_ACHIEVEMENT_INCAP_AFTER_SAFEROOM; sv_cheats 0
Apparently the achievement can take a few tries of this before it registers.
Pull your finger out Valve.
At least there's a fix.