The second was failing to immediately and forcefully apologize for that momentary loss of control, instead responding to the barrage of post-game questions in such a flip manner that came off as if he was trying to excuse or defend his actions.
The latter was the reason Montgomery's weekly press conference at Haas Pavilion, usually sparsely attended by no more than a half-dozen beat writers, was packed Tuesday. There were television cameras and radio reporters and unfamiliar faces interested not in Thursday night's vital Pac-12 game between the surging Golden Bears and conference leader Oregon but to pepper Crabbe and Montgomery with questions about that incident.
Crabbe continued to protect his coach, while Montgomery finally showed the necessary degree of contrition that had been lacking, the acknowledgement that would have kept the whole issue from ever becoming such a story.
While unfortunate, there was nothing wrong with what Montgomery did in that split-second as he misdirected his inflamed passions.
He did not grab Crabbe. He did not punch Crabbe. He did not choke Crabbe.
He did not act with malicious intent.
While dramatic, the shove wasn't worse than a verbal tirade or a finger to the face or any of the 1,000 other ways coaches berate, harass, intimidate, or embarrass players on a seemingly daily basis.
If Montgomery had apologized right after the game, it would have been dismissed as the product of the heat of the moment and nothing more. But Monty decided to be Monty, the glib, self-deprecating, half-bullying boss half-lovable uncle character that defines his interactions with the media, opening his very first response to the shove with, "It worked, didn't it?"
Doesn't exactly convey that he understands he made a mistake, does it?
Everyone in the room laughed, as would any Cal fan or player still caught up in the emotion of a dramatic win, but when scrolling across the ESPN ticket, it loses the context of the moment.
And when it starts scrolling across the bottom of the most important outlet in sports television, it becomes a real story.
Only then did the necessary damage control protocols begin, a statement released late Sunday night, and an interview with Pac-12 Networks on Monday, followed by the press conference.
Montgomery acknowledged that he made a mistake with that initial response, and that is the real mistake he made Sunday.
He had time to think about it and respond like an adult. Instead, he compounded his initial error.
This is a different time for coaches, where the limits of what is considered acceptable conduct have been dramatically restricted from what was the norm even 15 years ago. An old veteran like Montgomery should know that better than anyone, especially when these last years can solidify or destroy the reputation he has crafted over four decades. One misstep can destroy a legacy.
There are countless examples, and more often than not, it is the continuation – the failure to acknowledge and repent – that causes the downfall.
That should now be put to rest, but it never should have reached this point.
It wasn't the first mistake Montgomery made that has caused so much embarrassment for him, put Crabbe in an equally awkward situation, and distracted from the finest basketball Cal has played this season.
It was the second.
Dan Greenspan writes about the Pac-12 for Fox Sports Next. Follow him on Twitter @DanGreenspan.