I've been thinking the most serious crime we've seen by members of the Seinfeld Gang of Four is Jerry's mugging of the lady for her marble rye. Pursuant to New York State's Penal Law Section 160.05, that would be robbery in the third degree, a "class D felony." Thus, and assuming Jerry is a "first-time" felony offender(!), pursuant to Section 70.00, he would have gotten a cool two-and-a-third to seven somewhere upstate. (Of course, we can't have that; anyway, had he been caught he probably would have gotten to plea to a class A misdemeanor and been sentenced to three years' probation.)
What other crimes have we seen on our favorite show, by any of The Four or by anyone else either in a secondary (the families, Newman, Susan) or in a minor role?