Until you try and order something off-menu during the lunch time rush with a line of people out the door. Then, it's not so cool. Not for the restaurant employees who are slammed with orders, not for the people behind you trying to feed their pie holes during the 30 mins they have for lunch, and not for you when they make you wait an hour to get what you asked for.
So, here are some tips to keep your secret menu orders (or custom orders!) going smoothly:
1. Don't order off the secret menu or overly customize your order during a busy period (i.e. during the lunch or dinner time rush).
2. Be prepared to explain your order. There's no guarantee that your order-taker will know what a "Flying Dutchman" or a "White Gummie Bear" is (although the guy behind him might).
3. Don't bring your own ingredients. Seriously, it's against food safety laws for them to use it.
4. If they tell you they can't make it, there's always the self-help method i.e. order the component ingredients and make it yourself.
5. Not everyone restaurant is in the know, you might have to try visiting a few to get what you want.
6. Double-check your order. Whatever you're asking for isn't something the employees are used to making, so mistakes have a tendency to crop up.
And, remember, just because your local location doesn't have a secret menu doesn't mean you can't help make one. It's how secret menus get started in the first place...