When something doesn't work the way its supposed to.
I ran into these wore case nightmare scenarios when I used to do computer work. I have a hobby on the side doing A/C work now. Well I went to add some R22 freon to my friends A/C and the valve started leaking. There was no way I could stop it and wasted $100 of it into the atmosphere. On my own A/C unit I installed something called a 521 compressor saver. It's a capacitor that give the compressor a boost on startup which is supposed to put less stress on it. Well it worked fine for a day and then my UPS started switching to battery when the A/C would start. Then the A/C stopped working. The compressor overheated, the run/start capacitor I had just replaced burned out and the so called compressor saver exploded all over the electrical box. So I pulled it off and replaced the start capacitor. It was hook up correctly and HVAC tech swear by them. There is no reason why that should have happened.
Talk about stress. I guess I'm getting out of A/C work for the same reason I did computers