First, to the mommy that commented back in January I apologize for not getting back here sooner about Duck's sweet potato allergy. The first time he had sweet potatoes, Sam had a single streak across his face. A few days later we tried them again and he was head to toe with hives, his face swollen. It was the only new food he'd had all week and the only other foods he was eating were Nutramigen (hypoallergenic formula) and rice cereal.
Three years ago, we were in a buffet-style restaurant and picked up what had been labeled "pumpkin pie" for Sam and it turned out to be sweet potato pie. Thankfully we were around the corner from our apartment because by the time we'd gotten home, he had small hives on his stomach.
Sweet potatoes are not a common allergy, but it does exist.