1. Your fonts will default to the worst possible font available on the machine you are showing your work on.
2. If you have two versions of a photo, the wrong one will make its way to the printer.
3. The less time you have the more useless your computer will become.
4. Speed. Quality. Affordability. Pick any two.
5. Grammar checkers are a no no.
6. If three designs are shown to a client, your least favorite will be chosen or any combination of worst components of each.
7. If two designs are shown, a third will be requested. If provided, then one of the first two will be chosen.
8. If you ask for more copy it will be sent as a .jpg. If you ask for images they will send Powerpoint presentations.
9. Clients don't have their company logo in a usable print ready format so don't bother asking.
10. The best designs never survive contact with the client.
11. You will misspell the name of the client's spouse.
12. Your best idea is already copyrighted.
13. There is no stock photo ever made that matches the image you have in your head.
14. Time allowed to complete work is inversely proportional to time taken by client to work out what to complain about.
15. No matter how detailed the tech support FAQ is, nobody has ever heard of your problem.
16. The number of colors in a client's design will equal the number of colors in the original bid specs, plus two.
17. Your client will often not like your design but not quite know why.
18. Computer crashes always happen exactly 30 seconds before saving.
19. A client who knows exactly what he wants is worse than one that has no idea.
20. Clients who do not provide content upfront will complain about the use of Lorem Ipsum.
21. Everything has to be done immediately and deadlines are incredibly important, unless the client has to provide materials or approve your work.
22. The client will always think they know more about graphic design principles than you.
23. Your first price quote will rarely be approved.
24. You cannot force inspiration. It will only come when you aren't looking for it.
25. The customer is always right and an idiot.
I know there are at least 5 more rules that you would love to add to this list. If there are enough responses, I may compile yet another post of the "Unspoken Rules of Graphic Design!"