Let's say Luke mis-steps and accidentally kills his father during the fit he has when Vader threatens Leia. So he's on an anger-trip, and when Palpatine congradulates him, he kills Palpatine. Then he dons Vader's helmet, but doesn't make it out in time before the Death Star explodes.
And Yoda appears to Wedge Antilles, who is, after all, the chosen one.