1. You'll always be out of stock of whatever nut material your customer asks for.
2. You'll always blow the nut on a night before the repair is due back, with no more stock of that type of nut material.
3. The customer will always want the action at the nut the opposite of what you believe it should be. IE, you think it should be higher, they'll want lower and vice-versa.
4. You're best fret work will always be on a customer who won't notice the difference. Conversely, your worst fret work will be for someone who wants a super low action, no buzz, and can hear buzz that no one else can hear.
5. You'll always run out of a particular type of fret wire only right at the end of the job, and when the refret is due back the next day. Oh, and the customer will always want fret material that no one else uses, so you won't commonly stock it.
6. There will always be some type of finish contaminant when you're looking to meet a deadline.
7. You will always drop your spray gun and smash the tip just when you need to have the job complete and don't have time to order another one.
8. You'll always break a string right in the middle of a setup, feel bad about it, and eat the cost of the string instead of passing it along to your customer. (OK, maybe that one is just me)
9. Hot solder will always splatter on a nitro finish guitar and leave a blemish. If you completely cover the surface of the guitar, you'll notice it never splatters. It's like the solder actually knows when you are covering it. And, you'll cover and solder 20 guitars with no splatter, and it will lull you in to a sense of false security, so you'll slip up and solder with out covering the guitar. Then it will splatter and cause a finish blemish.
10. Hide glue will always stink worse when people in the house are already complaining about some smell from your shop. OK, if you don't have your shop in your house, this may not be the case.
11. Your best work will always be for someone who doesn't know the difference and is willing to pay double. Your worst work will always be for the pickiest customer you have, who'd like to set up a 360 month payment plan.
12. You'll always find a customer who sends the guitar back asking for more or less relief, and after months of work for free, you'll finally find out that the customer doesn't really know what relief means.
13. You can always count on Frank Ford and Dan Erlewine to make you feel like an amateur, no matter how many repairs you do.
At least, these have been my experiences....