The source engine is made by valve and been used for TF2, Left 4 Dead, Counter strike source, and other games.
http://en.wikipedia.org/wiki/Source_engine
http://en.wikipedia.org/wiki/Source_engine
Ok, so the source engine is a mixed blessing to the game world. Its the most moddable engine out there, countless game have been made for it, including Zombie panic!Source, Insurgency and Age of Chivalry to name a few.
It is often updated and fixed by valve for individual games and the engine in general, which means there are some slightly different versions floating around. But the high update and fixes make it a well maintained engine.
So far so good. But this is the problem. With every update, games performance takes a nose dive. With one fix, more bugs are uncovered meaning more fixes needed and more crashes occurring in the community. Its a cascading effect, Almost like a tree. You start with one thing which then branches of and creates more problems. Soon you have hundreds. A lot of these causing your TF2 to go from a healthy 35FPS during fire fights to 5 FPS during fire fights. Even the multicore rendering added to the engine is having mixed results so far.
So really, should Valve stop beating the dead and decomposing horse? Should they pump out a new engine for their next release? A more stable engine, still highly moddable, but with out the cascading bug and performance issues?
EDIT: To clear this up, I generaly have no problems with the source engine myself(3 cheers for tweaking) . I just picked this up and noticed it slightly on my own system.