What happened to me!
Let me end with a personal story.
OK, so you have read the list of ingredients and all seems fine. Nothing in there that should cause any trouble. But then you notice some smaller print at the bottom of the label.
"Produced in a factory also handling milk, gluten, egg and nuts."
What should you do now? Does this mean the food is unsafe?
Removing every trace of gluten from equipment used for other foods can be difficult. My personal preference is to avoid foods with such labels.
There is another thing to be aware of in the manufacturing process.
Some foods are sticky. Wheat flour helps to keep them from sticking together, or to the machinery. As this is not classed as an ingredient, it is not required to include it on the packaging. The quantity present in the food is minimal, but can still cause issues.
Talking of quantities...
Even if a normal sized candy bar is gluten free, don't assume that a smaller bar will also be safe. Believe it or not the ingredients can differ! Always read the label.