There are many reasons I do all my own spanner twirling, including but not limited to - cost/time/trust/quality.
Here is a small documentation of one of those reasons......
To set the scene - I was merrily driving my 1998 300tdi Discovery to a job back in January when, all of a sudden, complete loss of any forward motion and a little bit of a 'brrrr' noise happened. Engine dead. Because of the 'feel' of it I suspected the timing belt had given up. A swift couple of phone calls later (client to advise I wouldn't be turning up and the Automobile Association for roadside assistance) and following a bit of a wait a chap with a tow truck turned up, 'loosely' confirmed my suspicions and transported me and the Land Rover home. For anyone not familiar with them, here is the car:

That afternoon I had a bit of a look - took off the rocker cover, removed the rocker shaft and found these:

Well, look at that, every single pushrod is no longer straight....
So, thinking "I'll get on that soon" I needed a car to get me to jobs. Handily, I happened to have another Land Rover just laying about doing nothing so I convinced it to pass an MOT and put it into (almost) daily service. Now, this one had taken me to France when I moved there, then it sat in the garden (for 4 years) while we sold the house. The petrol in the tank was no longer burny, at all, in fact when I put a match to some it refused to light... Anyway, new fuel, charged battery, away it went. And here it is:

I digress somewhat.... Back to the point of the story.
Fast forward to earlier this week (yes, 'soon' turned out to be 8 months) I had more of a look at the Discovery engine. A couple of months ago I bought a complete engine (and some other bits) from a damaged Discovery and I'm quite glad I did as it turns out. Yes, I could have just swapped out the engine, but they are a bit very heavy and I have limited facilities so it was bought for 'parts' - reasoning that while it cost more than the £30 a set of pushrods would have cost, there might very well be more parts I needed.
So anyway, I took off the timing cover to see exactly what had happened. I was slightly shocked...
Belt:

The rest of the belt inside the case:

Camshaft pulley:

Injection pump pulley:

Tensioner:

Right, so all of those pulleys should be nice and smooth for a belt to run on and they aren't. There is a reason for that...
When a timing belt is fitted to these engines (which is the sole reason for removing the timing cover btw) it is tensioned using a dial/bendy torque wrench in a 1/2" square hole in the tensioner bracket. This hole should be nice and square:

That is not nice and square... Now, when I removed the timing cover 'something' fell out and hit my foot, here is 'something':

Yes, that's right, that is a 3/8"-1/2" converter. That has been rattling around inside the timing case making a mess of every (and I mean EVERY) moving part for the last 68,954 miles (belt replacement interval is 72,000 miles...) The last belt replacement is highlighted in my service history - which goes to show that a specialist service record means squat as far as quality of workmanship is concerned.
Oh yes, just for fun, because the idler pulley bearing was ruined by being hit about more than it really should be it meant the belt wasn't being held quite straight, which meant it could 'wander' about on the pulleys and do this to the crank pulley:

Soooo anyway, I then spent a couple of hours removing parts from the donor engine and putting them in mine, I set the valve clearances and did the timing:

Then, I sat down, had a cup of tea and steeled myself for turning the key.........................

Nice and smooth 740rpm idle :clap: Yay me.
So, to the crux of the title - one reason I never EVER let anyone else work on my vehicles for anything at all is that they tend to bugger it up in one way or another :neener: