The problem is that often there's an issue of why the NPC is there at all, and people's sense of what purpose they're probably serving. Just like TV shows or movies (and to far less a degree in books) you only have a limited amount of time to do what you need to do in an adventure, and while you may toss out some NPCs just for color, most are going to serve a purpose, and it can be hard for people with the right mindset not to spot the purpose. You see this in well crafted TV shows when the writers throw out a red herring or two (but often they aren't subtle enough to make it actually work reliably).