So today, I replaced my blade for the first time. I started shaving Christmas day and I have had a BBS shave ever since I began, but decided to replace the blade even though it hasn't caused me any irritation or diminished shave quality yet. Unfortunately, I was thinking about all the papers and reading I have due this week and so I was stressed and distracted. I cut myself three times. 1/4" lac behind the ear, a nick on the mustache line, and, here's the winner, a 1/2" laceration just under the chin. I could actually see the blade pass into the skin and bury itself there.
Lesson learned: Never shave while stressed or distracted.