Airline pilot here! (warning, do not continue reading if flying makes you uneasy).
One day we loaded up passengers and cargo and everything and we were ready to taxi. I noticed that the nose of the airplane seemed to be higher up than usual while we were sitting on the ground.
I told my first officer about it and he agreed. We double checked the weight and balance and everything seemed to be right.
I decided to just taxi out towards the runway and see if the wheel struts would go back to their normal positions during taxi. They didn't. Still felt weird to me. Something wasn't right.
I told ground control we needed to go back to the gate. Called ops and told them we're headed back because I think something isn't right with the weight and balance.
After we get back, I ask them to check how much ballast we have in the aircraft. It's verified on my sheet as 500lb, but I have a feeling…
Turns out, yep, they forgot to put it in the plane. So had we taken off, the center of gravity would have been out of whack – waaay past limits. It could have resulted in an airplane that was impossible to control. Just like that 747 that took off and had the load slide to the back.
That was a day that I was really pleased that I had so much experience flying to give me that feeling & that feeling could very well have saved my life along with others.
Edit: A lot of people are asking how 500lb could be an issue on a large aircraft. Not all airliners are that big. The incident I'm talking about in particular was on equipment with less than 40 passenger seats.