Of my 50 segments in 2014, 10% were late by your criteria. Two were very, very late because of the polar vortex in YYZ in Jan 14, (frustrating but understandable), one was late by about 4 hrs for mechanical reasons, (better late and fixed than not for a 6 hr flight over the ocean thank you very much), and one was to fix a problem with the oxygen masks that deployed when they shouldn't. One more was late because of heavy fog.