This morning I noticed something a little odd when I unlocked my bike; the rear seemed to bounce a little oddly on the ground when I moved it around. Upon closer examination I discovered, to my dismay, that the rear tire had gone flat overnight, under the influence of a rather large thorn wedged through the tread.
Thus I had to go back inside, change my shoes, and sadly drive to work.
I picked up a new tube on the way home (though annoyingly the first bike shop I visited was closed), and installed it without too much trouble.
Up until today I had never had a flat on this bike from a puncture. (I've had one or two from valve separations before, but those don't really count)
It's a good thing tubes are cheap.