Anyone ever been denied a security clearance?

Just want to hear some stories. We're you denied before bmt or after? I'm paranoid.. idont have a past but you never know. I know some old employers might put in a bad word. going to do nuclear engineer and I don't want to be reclassed into services or something I don't want to do for the next 4 years. I really want to serve in the air force as something where I can apply myself and challenge myself.