I'm usually not a paranoid person, but sometimes I get into moods of paranoia. Obviously this is more likely after watching a scary movie, but sometimes it just takes a little bit of thinking, and wham! Paranoia!
For example, whenever I'm walking a long distance and someone is walking behind me for most of it, I sometimes get paranoid that they are following me. So far, 100% of the time, they simply have been going to the same place I was. But still, hard to shake the thought.
Or, this past year when Sony, Steam, Rift, and a bunch of other sites were hacked, I was paranoid about all the little websites I have accounts on. Websites I've completely forgotten about, but probably have some User/password combo I use on a bigger site. I sometimes wonder if those little sites ever get hacked, and would lead to a compromised account on a bigger site.