I've managed to mount 700c tyres onto 27" rims a customer brought in one time. He said he spent quite a lot of money on the tyres through mail-order and didn't want to send them back to switch for the proper size tyres. So I loaned him some 18" motorcycle tyre-levers to mount the tyres. He managed to force the 1st tyre on, but it was completely distorted and the sidewalls were wrinkled and squeezed down between the rim-edges. On the 2nd wheel, he managed to bend and bow out the rim-edges enough that it cracked the rim.
I told him I'd be more than happy to sell him a set of 700c wheels to go with his new tyres...
