I used to waggle the wings of an old biplane I used to fly because an old engineer once told me to make sure the wave/flexing continued to the fuselage and even to the other tip. Seems if the spar (wood) were cracked the ripple would stop.
Well I was never too sure of the logic but I always did it so I feel confident that I never flew with a cracked spar !!!
Wonder what other funny/odd/strange habits we develop prior to flight ?