This, of course, is not always what happens. The whiskers, being very fine, don't have much current carrying capacity so they are quite likely to just vaporize. Nevertheless, there is risk. see http://nepp.nasa.gov/npsl/Prohibited/tin_prohibition.htm (look at catagory 3) you can get currents of 100's of Amps
you can get currents of 100's of Amps