We're not talking blatant headbutts a la Dominic Negus on Harrison, or Golota on Bowe in the second fight (though if you watch carefully, Bowe's head accidentally clips Golota right before and the latter obviously thinks he's been butted so he responds in kind. Of course casual fans and commentators always ignore these things or just plain don't see them). Though Tyson was definitely getting butted by Holyfield in both fights, that's for sure.
I have probably been as big a fan of Tyson as anybody, but I had little sympathy for his cry of foul play in those Holyfield fights. Tyson himself was a dirty fighter, long before he saw the inside of an Indiana prison cell. Watch the first Bruno fight. When ref Richard Steele is blindsided, Tyson cracks Bruno with an elbow to the temple that any Lumpinee stadium champ would have been proud of. It really hurt Frank, as his glove immediately went upto protect the area from being hit again.
Holyfield's butts weren't of the side swiping variety that people keep going on about. Tyson said it perfectly himself after the fight "He kept going down and coimng up with his head". It wasn't so much the shit he was pulling in the clinches. At 6'2 he would come in much lower than the 5'10- 5'11 Tyson, and as they got into punching range he would suddenly come upright with his head just BEFORE going into the clinch. He did this repeatedly in both fights, far more than I've seen him do to any other fighter. He knew that to beat Mike as a small man, he had to pull out all he had, including his dirty tricks. Tyson just got pissed off that someone would be brave enough to fight the "baddest man on the planet" in such a dirty manner.
I still think that Holyfield had enough without the headbutts to beat the 96-97 version of Mike. But he did cheat and land dozens of those ramming type of butts in those fights.