never say to a cop, 'man, i couldn't walk that line if i was sober.'
never say to your wife who's wanting to purchase a modest luxury item after she's pointed out all your toys (guitars, cars, video games, etc.), 'but, sweetheart, you've got plenty of toys already, like the washing machine, stove, vacuum cleaner....' i pretty much guarantee you they won't find the humour in that if my own personal experience is any indication.
when a woman complains, 'why do women have to have periods every month?' try to refrain from adding, 'are you sure it's just once a month?'