Here is just an example of one thing that can go wrong...
The guy in this Youtube video is obviously a "top level" valet. He's parking Lamborghinis like he's done it plenty of times before. But if you fast-forward the video to the 2:51 mark, you'll see that he steps out of a Lamborghini without putting the parking brake on... and the car begins to roll backwards.
He saved it!
However, that is a huge mistake, especially when dealing with expensive cars like these.
If a valet like him can make that sort of mistake, then every other valet has that same potential.
Just because you give your car to someone who seems to know what he (or she) is doing, it doesn't mean that bad things won't happen. All valets make mistakes. Even me.
The last big mistake I made happened when we were experiencing a rush. Suddenly a ton of people wanted their cars brought-out, and we were light on staffing.
I brought a BMW out, shifted it into park very quickly, shut it off, and began to run for the next car. One problem... I shifted it too fast. It didn't catch. The car began to roll forward. I hopped back in it and stopped it from t-boning another expensive car.
That was close! And it was embarrassing...
When I was a rookie at this, I did something similar during my first week on the job. I got out of the car while it was running and while it was still in drive. Oops! A supervisor saved the day.
I've driven manual transmission vehicles for 30+ years. (I had to adjust to using automatics.) Plus, they were bombarding me with information about how they do things, and I simply was distracted at that moment. It is not an uncommon rookie mistake though. I have seen it several times with new workers.