dave,
Done it all from the smallest glued-it-on-the-wrong-side mistake to building 2 left wings. If you do it, go calm down, then fix it and get on with the project. That's all you can do.
The best way to make sure mistakes don't happen is to be proactive. Cliches like "measure twice, cut once" and "check the fit dry before glueing" may sound corny to some, but it's a way of life on my bench. Read the instructions, make sure you're familiar with the step you're on and how it relates to the steps before and after. If you don't understand or need it, get help. Ask a pal or call a manufacturer's support number.
One more thing. If it's been a bad day, if you're angry or upset, tired or anything like that, DON'T work on your model. I've made myself a new rule that I don't work on a model when I'm working midnight shifts - too many mistakes (Chernobyl, Bhopal and the Exxon Valdez ALL happened on the midnight shift).
Don't rush a good thing. Enjoy yourself while you're building. It pays off in a better flying model.

papermache