Stress
This is actually a pretty stressful job, especially when you're first starting out.
You're a spy. You're playing a part. You're trying to follow a script and you need to store every detail that you see, hear, taste, touch, and smell in your head. If you forget a name, you're going to have to think of a way to ask for it again without being suspicious.
If you leave out important details for the shop that are asked of you or forget to ask for a receipt, chances are you won't be paid a dime for all of your efforts. (And yes, it happens to everyone at one time or another.)
Most stressful of all is the chance of getting caught. It's the cardinal rule. If you get caught, it makes you and the company that hired you look bad. Plus it's super embarrassing if an employee realizes you're snooping on them.
Don't get caught.