5 Biggest mistakes made in UFC title fights
#4 Cain Velasquez vs. Junior dos Santos 2 and 3
Dos Santos entered UFC 155 as the UFC Heavyweight Champion, however, the biggest error he made in the fight turned out to be the fact that instead of circling out of harm's way, JDS would move back in straight lines upon being pressured by Velasquez's strikes. That, in turn, opened up opportunities for takedowns, as the latter repeatedly took Dos Santos to the mat and wore him out.
Velasquez beat JDS from pillar to post, and ended up winning a Unanimous Decision, thereby recapturing the UFC Heavyweight Championship which he'd lost to JDS back in 2011. Furthermore, their trilogy fight took place at UFC 166 in 2013, and went much like their second fight -- except this time around Velasquez managed to stop JDS by way of TKO in Round 5.
Dos Santos' mistake of moving back in straight lines, and abandoning the disciplined footwork he showed early in his career, cost him dearly in his 2nd and 3rd fights against Velasquez.