I can't tell you how many times it's happened; I was positive I hadn't eaten any dairy then those all-too-familiar symptoms started. And to top it off, it always seemed to happen at the most inopportune time, like a business dinner.
After a lot of research I've narrowed down a few of the foods hiding lactose and dairy products. This list is by no means complete and I'll keep posting others.
The best way to be sure is to check the ingredient list. Oh, and take it from my own personal experience, you'll want to check the ingredient list everytime, even if you're picking up your favorite foods. You never know when a manufacturer will change their recipe.