Saw this happen once on a flight I was on. They found a loose bolt during pre-flight checks and delayed us for hours. It was annoying but better than risking it in the air. Now I always respect those safety rules, even if it means waiting. Small things can lead to big problems in aviation.
My uncle works on the flight line at O'Hare. He says they find loose hardware all the time and it's almost never a critical part. The checks are important, but a single bolt rarely means the plane is unsafe to fly.