Yep, me too. Just a reflex reaction to use my index finger to hit the lock button. How i hold my phone differently it'll take a little while to readjust, it's not a "design flaw" or "a problem" it's just human nature.
When human nature causes an unexpected behavior to happen to a carefully designed device (some would say the best designed in the world), then I think we all agree it's a design flaw or problem.
Isn't that really the definition of a design flaw?