When I filled out my first SF86 13 years ago I left out two details. 1) I stated the number of times I had smoked pot as less than it was. 2) I did not include that I one time sold a ritalin pill to a friend (pills prescribed to me and it only occurred once)
At the time I didn't understand the main importance of just being truthful and had been scared by an interview with a different organization. Soon after filling out the original forms my clearance was upgraded and I left the information as first reported.
I have been reinvestigated since but the time period no longer covered those things and so they remain unreported.
I'm about to have my first CI poly. and it is concerning me that this may come up. I plan to be entirely honest but don't know if I need to proactively say something.