I felt sure someone would have come through with that old proverb, but I've skimmed through the comments and don't see it, so here it is: "The road to hell is paved with good intentions."

So I looked it up and apparently, this expression was used by the writer Samuel Johnson and is mentioned in his biography in an entry dated 16 April 1775, in the form `hell is paved with good intentions'. The idea is that good intentions do not guarantee a good outcome.

So there it is. Someone's mentioned it now.