By far the best example of this is liquids. Going through security you see a great big garbage bin filled with fluids tossed away. Under the guise that they are potential explosives.

Now what would happen if someone tried to bring *actual* liquid explosives through in a water bottle? They would get tossed in the bin with the rest, and no one would give it a second look.

So now you have a situation where either we have to treat every single liquid filled bottle (the vast majority of which are harmless) as a live explosive (shut down airport, evacuate, bring in bomb squad, etc) or treat every single liquid filled bottle, as well a water filler bottle. The former wastes huge amounts of resources, the latter completely discounts the threat of actual explosives.