A couple of months ago, I was also returning from Thailand into SFO... and was randomly selected for customs screening. The agent did the normal look through my bags (which found nothing interesting) and asked for me to turn on my laptop. Fortunately or unfortunately (depending on your perspective) my laptop was on its last legs and was taking forever (like > 10 minutes) to boot. I was very patient and just told her that it might take a while... I was at my final destination for the day, and it was too early for me to check into my hotel anyway, so I decided that I had time for this.

As the laptop took its own time, I was just hanging around, and the agent was getting a bit frustrated... Finally, I got a "never mind" and was told to go on my way.