Friend, lover, team-mate or mother, you think you're on a wavelength but you're not.
You might be in the middle of a conversation about the European Central Bank, or an excellent new cat video when suddenly:
You make a turn it was obvious you both should take; or
They make a turn without warning.
All of a sudden knees are touching, brake levers are scraping along knuckles, someone is squealing like a macaque and if one of you doesn't hit the ground soon, you both will.
BONUS TIP: The remainder of the ride will feel like an 800 mile epic through a landscape of frigid silence, punctuated with sightseeing stops at the sulphuric geysers of fault and blame.