Same thing happened to me, if ur wheels permit, grind a slit in the nut, grab a cold chisel and a shifter. Stick the cold chisel in the slot but dont grab the shaft of teh chisel (shifter will just open, spanners will snap etc ), grab it fairly close to the tip where its flat. also we used a piece of wood to lever against the back of the chisel to stop it slipping out of the groove. also if its the last nut on the wheel usually if the wheel is off the ground u can get about 1mm movement by wobbling the wheel back and forward, every bit counts.