I didn't realize this. If you live in Michigan, remember this on Tuesday:

Thousands of votes may be spoiled (Lansing-AP, August 4, 2002, 6:33 p.m.)
Hundreds or even thousands of votes cast in Tuesday's primary could end up being tossed out, if voters pick both Democratic and Republican candidates. State law bars primary voters from casting both Republican and Democratic votes on the same ballot. But election experts say voters do it anyway. Experts say voters cross party lines either because they don't know any better or because they think they can get away with it. Michigan voters have a history of ticket-splitting in general elections, but may not do it in primaries. The idea is that each party is able to put its best candidates forward and that folks of like-minded political orientation will make the decision. The system also discourages partisans in one party from casting ballots for weak candidates in the other.