We had a mini-earthquake (at least, I assume that's what it was) which startled me. I immediately began having daydreams about what would happen if a giant earthquake or volcano occurred; where I would go, what I would do, etc. Lost the count.
Picked the count back up again, and focused on my breath, which was still coming quickly. I felt anxious; I wanted to move on to the next breath, and the next, and the next. I was barely paying attention to one because I was busy anticipating the next.
At one point, my AIM client chirped. [I had evidently forgotten to set my away message.] I wanted very much to stop the session, get up and see who it was. I observed this impulse, and the tightening in my chest that came with it. It passed in relatively short order.
Gradually, my breath and heart rate slowed to normal levels. I began to microsleep periodically, losing the count a couple more times. At one point, I snapped my head up to realize I had fallen asleep for several whole breaths. I sat up straighter, took a deep breath (which I used as the first breath of a new count). My breathing came slightly faster, but then I started to drift a bit again.
Then the bell rang, I completed the set, and stood up.