(This is yet another interpretation of Murphy's Law. I think scientists need to seriously start considering Murphy's Law as the Grand Unified Theory of Everything in Nature.)
In any given office setting, no matter how well you perform your job, there will always be someone who sees only your mistakes. Events and timelines will conspire to ensure that your superiors are present when this coworker decides to draw attention to your mistakes. The situation will also be such that your defense would either seem silly in the context (as it would sound defensive and defeat the purpose) or would take up too much time and, well, see the first part of this paragraph.
Usually I can shake this off: I'm good at my job and the folks that need to know it, know it, and they know it well. I'm not perfect and welcome the opportunity to see where I can improve my performance. Also, some people will just never like you.
When I encountered this reality last week, I did shrug it away. But when it showed up again this week early in the morning before the caffiene had kicked in to shore me up after a terrible night's sleep...I let it get to me. Just enough to make me vent to Mark on the drive home.
I have a feeling I'll be over it tomorrow. Futile office politics just aren't worth the mental energy.