It may have also been that Yoda was looking far into the distant future relating to the effects of
Han and Leia's offspring
i.e. the rebirth of the dark side and the galaxy's next largest threat... That being said, this is all opinion and it was never officially explained so it is anyone's guess as to what he actually meant. Also, it is possible as Yoda said "Always in motion is future." It may be that certain things that were meant to happen didn't and that many events were changed (as examples... Mara Jade, Kyle Katarn, and in fact all the extended universe as it was told may have been altered in that moment...), this would explain many of the offshoot stories of the Star Wars franchise without creating so many of the contradictions so many fans fuss over.
For that matter it is implied that in several ways the force binds all across all time and there fore changes in the now or future could even effect the past... which if you want to appreciate the new versions is probably the way to do it. This perspective is likely not what Lucas meant as there were little to no plans beyond these first three movies, but as far as accidents go it may very well be the one unintentional meaning that binds the Star Wars universe (entire collection) together.