Over the years in my trade I have seen "qualified" tradesman come and go through the company I used to work for.
Here's a list of the worst jobs "qualified tradesman" I have seen.
1) Foxtel tech installed a foxtel point in a toilet simply because it was the room behind the cable entry.
2) mount a power point to the underside of a spa only to find the screws penetrated the spa bath
3) mount a power point in a cavity needed for a sliding door
4) cut a hole for a power point only to think he couldn't get a cable there, cut another, then another, then another. After 4 holes gave the customer a bill when he did nothing bit destroy a wall. The wall was an internal wall with a noggin. He should have known better and drilled down.
5) screw a ceiling fan to the ceiling gyprock with no supporting timber.
6) mount a split system A/C head unit to the wall but not screw it to the timber (this was Marsh Air at my first house)
7) Install an outdoor light in front of the back door so the door couldn't be opened
8 ) Install an IXL over a shower. hmmmm hot glass, cold water!
9) Try to use a hammer drill on a glass splashback and wonder why the glass broke