In the millennia humans have been sailing, all of these have happened.

Was it intentional and repeatable?

Lots of examples of this, of course.

Is it even possible to make the trip in both directions? Or did they get an assist (like an ocean current) that pretty much ensures a one-way trip?

If the Polynesians-in-California scenario actually happened, that was most definitely a one-way trip, assisted by currents.

Or did they experience the equivalent of a missjump and being forced to homestead in a new location due to not being able to go home?

Pitcairn Island offers a very interesting variation on this theme. Another similar case is the Mayflower, which was forced to put ashore and found a colony hundreds of miles north of its originally planned destination. Roanoke is also in this general category.

Or were they, effectively, political or criminal exiles, kicked out or fled from their homes with whatever they could pack on the boat, and facing the threat of execution or worse if they ever went home>

Australia, though in this case the Brits thoughtfully provided transportation. :) Many immigrants to the New World were in this general category as well. E.g. the Mayflower colonists were fleeing religious persecution in Europe, having already worn out their welcome in Holland and Britain.

Craig Berry (http://google.com/+CraigBerry)
"Eternity is in love with the productions of time." - William Blake