I am thinking of things like:
-A single punch that bailed a fighter out, and maybe the loser never recovered from.
-A ref stopping a fight.
-A cut or headbutt that changed a fights outcome and eventually a career.
The biggest 1 minute (that I can think of) that changed the next 15 years and probably all of heavyweight boxing forever was between round 4 and 5 of Clay-Cooper. Did Dundee rip Clay's glove? If he doesn't get that extra time does he lose by TKO early in round 5? If Clay loses that fight he doesn't face Liston 6 months later, and that would have had a huge impact on his career. Who knows how many more fights he would have had to win to get a title shot. Liston may have reigned for another 2-3 years and he could certainly have found his way on to most top 10 all-time heavyweights.

Another instance was the refs stoppage of the Taylor-Chavez fight. I am not arguing wrong or right, I am just saying that was an enormous split second decision. It allowed Chavez to keep his streak alive, and in turn he kept it going for another 20 or so fights.
If it doesn't get stopped, Taylor wins the decision. He was never the same after that fight. Granted he did take some punishment, but his career going downhill must have been mostly mental after that loss. Some fighters never get over being "robbed" (whether he was or wasn't). If he wins that decision who knows how long he could have held the title? Could we have seen him in a showdown with Pernell Whitaker a couple of years later?
I am sure there are far better ones, but those just came to mind as I was watching Taylor-Chavez Legendary Nights today.
Let's hear some good ones....