I just began playing Super Meat Boy on Steam a couple days ago, and I beat Dr. Fetus earlier today, but I never got the achievement for beating the light world. I have logged out of Steam, quit Super Meat Boy, and got back on, but the achievement isn't popping up for me.
It's really not that big of a deal, but I do like collecting achievements, so hopefully I can unlock it sometime. Has anyone else had any problems with achievements not unlocking for you? It's sort of funny having my 90 bandage achievement / Cotton Alley achievement, but no beating the light world achievement...
My main assumption is that I wasn't connected to the internet at the time I unlocked the achievement, so I'll probably just beat Dr. Fetus again, and see if I get it then. http://steamcommunity.com/id/Vaati33/stats/SuperMeatBoy
edit: I replayed the Dr. Fetus boss and that solved the problem.