If you eat any food that has a high sugar content you are likely to dump (which means getting hot, feverish, sweaty, like a "hypoglycaemic" episode -- at worst it can lead to vomiting as well -- many bypassers feel the need to lie down and let it wear off if an episode happens).
This happens because your tiny stomach and re-arranged intestines have very little capacity to deal with sugar.
For some bypassers this happens with meals that contain a very high carbohydrate content, as well as sugary/sweet foods.
Am not sure I am giving the best technical explanation, just a brief layman's explanation, and possibly someone else will come along with a link or a slightly better explanation about "dumping".