I don't have a security clearance but one day in the middle of the night I accidentally ended up at the Belarus/Polish border due to a faulty bit of navigation software (Thanks, TomTom). I'm pretty sure that that had absolutely no effect whatsoever (I had no visum anyway and it's not like you just waltz across that border, nor did I want to) but at the same time if I would have a security clearance I'm fairly sure that there would have been some effect. When you are operating at that level you should be a lot more careful about who you are buddies with and where you visit lest someone decides that maybe it is time that you lost that clearance.