Pretty much what it says on the tin. My inventory says that I’m carrying 55.2 weight and have a max capacity of 134.5, but any time I pick something up I get the “You’re overburdened!” warning. However, the speed penalty for actually being overburdened is nonexistent in the stats screen.
Looking at the commit log it looks like the weight system was just overhauled, so my guess is that’s the culprit.