Oh, man, I sure didn't like mask clearing the first time I had to do it. Now? No big deal.
My big goof when I was doing my first certification dive for deep class was that I didn't dump air as I ascended. Well, turns out all that stuff they tell you about the pressure of the water is true! I needed to inflate some at 80" in order not to bottom out. Well, get to about 20" to get ready for the safety stop and what happens to the air in my BC? It's under a lot less pressure and up I go. Grabbing for my dump valve, I end up with my gauges in my hand and break surface. Note to self, courtesy of my instructor: release air gradually on ascent from a deeper dive so you maintain control. (And configure your stuff so you don't reach for this and end up with that.)
My instructor is a level-headed piece of work, and I knew enough to dump air, go right back down and do the stop. But I am thankful for good instruction and a safe environment in which to goof now so I don't do it later.