For me, it was when I was first learning to squat clean. I had 225 or 102 on the bar and I was set to do doubles. I was doing two sets. On the first rep of the first set, I caught the bar a bit high and rode it down. Then somehow my weight shifted a bit and I fell backwards. Luckily I had the presence of mind to shove the weight away from me before I got trapped by the bar. Ended up on my duff with a red face.
Not to admit the defeat or the embarssment, I immediately stood up, grabbed the bar again and proceeded to my second rep.
The 2nd rep was even less graceful than the first.
It was awesome. Unfortuantely, I didn't record it. I wish I had. It was realllllly embarassing. Oh, and I skipped the 2nd set. As it was the Y - a larger gym - and it was pretty busy that day. One of the trainers came up to me afterwards, and he's not afraid of me in any way despite my ogre like appearance, and commented "so, you're not quite grasping 'the squat' part of the 'squat clean' are you?"
We had a good laugh and it was all good.
What I learned - use lighter, completely controllable weights when you're learning a complex movement.
Next step - my OCD personality pretty much speaks for itself. I still don't feel completely competent in the movement, but my confidence has increased dramatically. I've gotten to a point where I feel I can squat clean just about whatever I can front squat - but I've not tried it, I just feel that way for some reason. No way in blue hell I can jerk that now, but that's only part of the process.
All the best,