When I go out to a restaurant, I order gluten free food items.  I have to trust that the cook and server take my request seriously.  If not, my food could be "gluten free" but cross contaminated with foods that aren't and still make me ill.
If it's mild, just stomach pains for a few days.  If I were to say, eat a bun that was supposed to be gluten free and it was full gluten, my joints also inflame.  So you see it can affect more than just my stomach!
Inflammation is the devil.  I'm just glad I found out how to manage it to improve my quality of life.