I could be wrong, but I think that actually happened back in the Fred Akers era.
Haha, my daughter goes to UT but grew up in the Waco area. When I mentioned this possibility to her she was absolutely horrified. To the Longhorns this is a fate worse than death. I can't remember what year it was, but several years ago we went to the Baylor/UT game at Floyd Casey stadium in Waco that Baylor won. The Bears brought down the goal posts they were so thrilled. It took weeks for them to come down from that high. If they beat the Texas in Austin they will be on a year long high.
Losing to Baylor was a fairly regular occurrence in the ‘80s and ‘90s.
David McWilliams’ teams lost to Baylor *in Austin* in 89 & 91.
Mackovic’s team lost (at Baylor) in ‘97... I think that was the last time the Horns lost to Baylor.