Paying attention

I was walking down one of the corridors at work, and a little rattling sound nudged me out of my preoccupied thoughts. I stopped. Silence. I started walking again and the rattle resumed.

I got to my desk, took off my shoes and turned them over. The rubber had worn away a bit on the sole of each, leaving a little flap over the heel's hollow core. I stuck a finger under the flaps and, no lie, extracted about eight pieces of gravel that had managed to jam themselves up into the heels. I put the shoes back on and the rattle was gone.

Tell me about some minor irritant that you suddenly noticed, and when you took care of it you wondered, "How come I never noticed it before now?"