I've been on 9 flights that I can recall over the last 25 years that aborted landings. 6 at YYZ all because of either another plane still on runway in front of us or because of birds(geese) being hit by another plane on it's takeoff and the maintenance crews had to go clean up their remains off the runway. Best one of these was on a BA 747 when we were almost on the ground when the pilot just gunned the engines and we went round(the thrust was amazing ,much more noticeable than on a conventional takeoff). 1 at MAN on a 737 due to wind shear and that was scary the way the plane was being tossed around prior to aborting. The other 2 have been at LHR due to another plane still on the runway and PIT for the same reason.