I've done it a lot over the years, sometimes on purpose, sometimes by accident. In the heat of jacking off or fucking or getting a handjob it happens. The best though was the other night. I had the tube on and my wife was working on me with our Hitachi Magic Wand while we were setting on the couch watching TV. My feet were on the coffee table with my back bent forward enough that the head of my cock was about six inches from my face. I started to cum and just then my wife grasped the tube in her hand and pushed it down hard against my pubic bone. With that I exploded and shot a huge load into my face mouth and eyes. My wife laughed as it dripped down my face and she smeared it around until it was a sticky white mess.
I guess the combination of the compression caused by the tube, the vibrator, being super turned-on and my wife pushing back on the tube a just the right moment to release the pressure caused the explosive effect. It was great fun because it was a complete surprise.