This was one of the first big fears that people had about GMOs. If you know that you're deathly allergic to peanuts, you'll probably stay away from peanut butter. But if a scientist puts peanut genes into some other food, would you have to avoid it as well, and how would you know they were even there?
There actually is an example of this happening. Back in the mid-1990s, scientists found that a Brazil nut protein, added to improve the nutritional quality of genetically engineered soybeans, had the ability to cause an allergic reaction in people sensitive to Brazil nuts [source: Nordlee et al.].
Unwary consumers never actually got sick from eating those soybeans. That's because the researchers discovered the dangerous effect during the development process and ultimately abandoned that particular GMO [source: Palmer].The allergy risk could be nipped in the bud by such safety testing, which is why the American Medical Association, among others, has called for mandatory pre-market safety testing of GMOs [source: Eng].
The FDA doesn't require allergy screening, but instead uses a "voluntary consultation process" to look at safety. And while the agency hasn't found any GMOs that cause allergy problems so far, it's possible that some might slip through the regulatory cracks. The answer might be for Congress to mandate such testing and provide funds for it, but don't hold your breath waiting for that to happen.