At UFC Fight Night 38, Scott Jorgensen took on Jussier Formiga in the preliminary portion of the card. After a few short exchanges, Formiga dropped Jorgensen, took his back, and latched on the fight ending rear naked choke.
At first it seemed that Formiga had dropped Jorgensen from an uppercut or some other punch, but on the replay it was revealed that Jorgensen was dropped by an illegal headbutt. While the headbutt was incidental, it was certainly illegal, and likely led to the finish for Formiga. The referee did not see it, and thus Formiga was given the submission victory.
Formiga believes otherwise, stating that no headbutt ever occurred (Courtesy of MMAFighting):
I don’t think it was a head butt. I moved forward and my head hit his chest and he went down. I took the opportunity and took his back. I wanted to go for a leglock, but I lost the position and went for the neck.
As of now there is no word whether or not Jorgensen will appeal the decision.