3. They never wear sunglasses outdoors.
In movies, agents always wear sunglasses and black costumes. Real agents never use sunglasses during their mission as they might miss something important because of glare or lack of light.
2. Sometimes they fail too.
Nowadays it’s really difficult to hide anything, and a Secret Service mistake is no exception. There was a story of Barack Obama riding in an elevator with an armed officer when a man with a knife entered the grounds of The White House by jumping over the fence.