there are always cheap customers that dont want to repair anything.
Most of my calls are to change the filters and change belts because the evap was frozen solid. and in every invoice i state " Requires rountine maintenance" Stupid customers but... they pay the bills when it happens!
Ive seen Coolers working off 404a which i was told works better and cheaper now adays
I can see adding different refrigerants to systems by mistake all the time. I got a habbit now to bring sprays can with me and paint the colour on the head of the compressor or something accessible.