It cost me £6000 when my friend wrong slotted my 911 with 915 gearbox, he changed hard at around 120/130mph from 4th to 5th, it actually went in to 3rd and buzzed the top end of the engine, pretty much all the valves met all the pistons. He was at fault when borrowing my car but he could not afford to fix it....he made it all right again when he had his own body shop, I got a few free resprays out of him.