I was on a Trident
way back when on a pressurisation check.(On ground of course) We were at max diff checking the first safety valve when by esteemed colleague let go of the wrong switch. This resulted in popped ears a dense white fog and the airframe fitter appearing through the fog with the safety valve collar in his hand ready to clout said esteemed person on the bonce. Switch set the other way and popped ears again! (Yes you do get white fog!-and red mist....)