reasons that have caused me to focus a board:
temperature, cracks, rocks, loose trucks, tight trucks, old shoes, new shoes, wind, 360 flips, etc.
luckily it hardly ever happens anymore, i have focused maybe 2-3 boards in the past few years. i usually just take a break if im getting pissed/frustrated.
i've definitely focused a board that was only skated for 1 or 2 sessions...whoops?