I have seen two cracks, a hole, and an open panel.
One crack was on the engine nacelle and the flight was cancelled. The hole was on top of the fuselage, at night, and nigh impossible to see; te flight was cancelled.
The open panel was seen in flight. It would detach or it wouldn't. The consequences would probably be insignificant but if reported might have caused a turn back in mid-Atlantic; the difference would only have been an hour or so. I made a decision not to tell the crew.
The other crack was on a wing flap. It was not spreading although it was flexing with slight turbulence. I reported this to CC but their knowledge was less than mine and they simply asked me to remain in my seat after landing and point it out to the captain; they did not seem willing to take responsibility. When the captain saw it she thanked me and said she would call the engineers.
On much earlier flight my father said he had watched rivets popping on the wing. As a master mariner he was quite used to rivets popping on his ships so thought nothing of it. The aircraft was G-ALYV.