Even the professionals have their troglodyte moments:
I read in a bike magazine of a shop mechanic who, acting upon a customer's description of "a rythmic clicking sound when pedaling hard" removed and replaced the bottom bracket on said customer's bike...
... Only to hear the same noise when he gave the bike a test ride.
Next, he looked in the seat-bag, and found a 6" open end adjustable wrench. It had rubbed through the bag fabric, and was tapping the seatpost... "rythmically, when pedaling hard."