Has happened to me both in a PA28 and a C172RG - gets a bit draughty, but otherwise a non-event, at least in these two types. The door simply trails slightly open. To close, you typically close the vents, open a window and pull it shut. Pushing it out a bit first helps. However, if you're alone (and don't have an autopilot), it's probably better to simply leave it and land as soon as practical.