Oh lots, how much time have you got 
Gave up college because my father was unwell. Should have stuck with it, will never have the career I wanted.
Left home at 18 and lived with someone 11years older. It didn't work out. Should have stayed home and stuck at college.
Wasted myself and my health working 70hrs a week in work that doesn't suit my character.
Had children but in truth was never really cut out for it. But made the best of it, and put everything into it. No savings and no career.
Got educated at 40, good degree but wasted it because could not make "big girl decisions" so again stuck doing a job I'm good at, but stressed by.
Remarried, DH is gorgeous but difficult. Very romantic, emotional, caring but god does he talk. He's never quiet. In retrospective I'd be more content with an introvert like me.
Move house to DHs home town. Very pretty, very arty, very liberal but now over run with posh hippies campaigning to ban cars and dig up the roads. It's beautiful but it's miserablebin equal measure.
Most of all I regret not making big riskier decisions and the fact I've never had the confidence to push myself.