My first clearance interview, about 10 years ago, it was all going along just fine. I had the standard litany of "bad" things any early 20-something American guy gets into - a few mushroom trips, an alcohol ticket, friends from foreign countries. No deal-breakers so long as you are honest and I answered honestly.

The only stumbling block came when the interviewer asked "have you smoked marijuana?". I truthfully answered no. The interviewer suddenly changed from bored old lady to the hardened, ex-cop that I suspect she was, glared into my soul and asked again. "No", I answered once again. "Well, that does not check out with your background. We will have to ask around on that one." She did ask around, my friends corroborated my story and I got the clearance.

My background involved undergrad education at a #1 party school, in a college town where open marijuana usage was common well before being legalized. And I had casual experimentation with other drugs (the aforementioned shrooms) and plenty of alcohol usage. I was probably the only one in my college acquaintance circle that didn't smoke on a semi-regular basis. I sometimes think I could have lied and said "yes I smoked weed", and still gotten the clearance. It would have actually been less of a red flag for the investigator(s).