Hi! I've got an irresistable urge to share this.
cleans eclipse caches and can make a very stubborn build error go away.
I've had a "resource already exists on disk" in a m2eclipse driven project. Deleting the file manaully/refresh/clean/close-open-project did not help. workspace/.metadata/.log contained a lot of ClassCastException-s.
But closing Eclipse and re-running it as "eclipse -clean" did help.