The parking wardens will have to be present and will confiscate
the tampered coupons as evidence for court. So, if you see wardens
standing outside the car, jump in, start the engine, and eat the
coupons immediately, and drive off. 
The fine is not fixed, but you will have to haul yourself to court
and the fine will be determined by the presiding judge. It will be
several hundreds.
Don't do it. Come on, you can tamper in only one of two ways:
either fold back or re-stick. What other tricks have the wardens
not seen before?
