In the beginning of "Tornado Prom" (season 4 episode 15) Kelso says "Hey, guys. It is literally a million degrees out. I'm wearing shorts in January! Have you ever seen prettier legs on a fella?" The rest of the show is based on a Tornado going through town and is at one point seen behind Eric as he is driving along to pick up Donna at the radio station.

Problem is there have been three January tornados in Wisconsin; One in 1844, one in 1967, and in 2008. There were no January tornados in the 1970's.

See quote from episode:

http://www.twiztv.com/scripts/that70sshow/season4/that70sshow-415.htm

See USA Today story about January tornados in Wisconsin:

http://www.usatoday.com/weather/storms/tornadoes/2008-01-08-wis-tornadoes_N.htm