Cold.

Rear tire was flat when I got to my bike locker yesterday evening! It had sat for a couple weeks, so I didn't know how fast it was leaking, so I pumped it up and risked it. Had to pump a couple more times on the way... no big deal. Turns out the tube was defective; it was opening up at a thin spot. I patched it anyway

Also, my tail light had stopped working. I turned on the spare tail light for riding home, and checked it out when I was fixing the tire. Turns out the soldered connections on the wire had corroded away; the wire was completely gone in one spot, and when I touched it it crumbled at three other spots. Hiding wires under the fenders makes sense from an aesthetic point of view, but it exposes them to pretty harsh conditions, including concentrated salt water. Ugh.