Only reported an issue once in 20 years...
Like many on this thread, I've a few million air miles under my belt but did report and get a B777 turned back just before the runaway in Houston one time. I noticed that with the flaps up there was a large black rubber looking "thing" sticking up out of the rear of the wing. By the time I had pushed the call button, the Captain had lowered flaps for take off as he was taxiing out, and the rubber had vanished. The cabin crew who attended asked me if I though she should call the cockpit! I suggested yes, since as a private pilot myself it was fairly obvious things shouldn't stick up out of the wing and whilst I may only have been a rubber boot for the flaps, I have no idea what that may have done flapping around at Mach 0.85. The co-pilot to his credit came down the back whilst we were still taxiing, but couldn't see anything. I suggested they bring the flaps back up to see if it re-appeared, and fortunately it did. They turned the plane round, and got an engineer to glue it back in place. 40 mins later we were on our way and made up the time on route back to the UK. So, in the end, happy I pressed the button. Most likely it wouldn't have mattered, but since I'm not the bloke at the front, I'd rather let them take an informed decision when something was obviously wrong.