Not so much when I first started riding, but I have been guilty of uttering the following over many years of riding:
I never fall on my head why are helmets now compulsory,
Suspension forks are only for soft downhillers with no skills,
Who needs 7 speed,
Who needs 8 speed,
Who needs 9 speed,
Real XC riders don't need full suspension,
Clip pedals are just an accident waiting to happen,
Steel is the best material for mountain bikes,
Air forks/shocks are unreliable,
Disc brakes are overkill,
Bar ends are useful,
1" of travel is heaps etc etc
etc
etc
etc