Yup!! Done that one, more than once...first time was when shooting some stuff for someone (had a backup 35mm of all the 120's) right after I got the camera. 2nd time was after I had not used it in a while and sure enough. It seems like it should be hard to do, but somehow it isn't. All I know is, when I take the film insert out of the camera when a roll is finished, if I see the color of the paper (Ilford=white, Kodak=yellow,etc) then I know life is good, if all I see is black...as in the back of the backing paper, well then I know I better have some backup or plan to re-shoot.
Sorry Sean....I know it stinks, but bet you don't ever do it again....or maybe once more.