Do plain canned vegetables require a hechsher What could be wrong with them?
A. In past decades it was fairly safe to buy certain vegetables without a hechsher. A typical cannery would run only one kind of vegetable and close down during the off-season. In more recent years, canneries have tried to remain open year around in order to maximize profits, and so the likelihood has increased that canned vegetables are heated in equipment used for other products as well, some possibly non-kosher. Many canneries, in addition to their vegetable production, will process non-kosher products such as pork & beans or cheese sauce. A mashgiach once walked into a plant that was supposedly dedicated to a single vegetable and found that in the off-season it was producing shrimp soup and alligator soup!
