I'm always amazed at how nonchalant people are while approaching a challenging weight. If you wait until you've stepped out of the racks to get serious, it's already waaaaay too late. Set-up and proper breathing are crucial to a solid squat with maximal or near maximal weights. Of course, these are skills that need to be practiced and automatized if you want them to be there "when it counts".
While shooting Squat Rx #10, I missed a squat with 405lbs - not something I'm proud of as it's not particularly heavy but I wasn't in great "squatting shape" at the time. I thought about editing it out, but decided to include it anyway - it didn't seem honest to leave it out completely.
I added the text to the video later while editing because I felt I hadn't explained the breathing properly. If I were to re-shoot this, I'd make sure to talk more about diaphragmatic breathing in detail.
I've talked about breathing a little over the years here on the blog and this post, Squat Rx: Posts on Breathing, has links to most of them. This article, Improving Squats With Diaphragmatic Breathing and Voodoo by Jim Laird, is a great resource as well. Enjoy!