They say no matter how many games you have watched, you have never seen it all in baseball. This play last week was proof of that. There is virtually no way to discover if this has ever happened before either, as it would not be recorded in the play-by-play data.
This does remind us of the odd rule—or lack of a rule—concerning two runners occupying one base. Which is that the runner is not out until he is tagged.
Therefore if the situation were to arise that the defense refused to tag out the trailing runner, and instead pitch to the next batter, both runners would legally be occupying the same base. Unless the umpires stepped in, and we’ll assume for fun that they do not, all nine players on the team could theoretically be on base simultaneously.
Or just shy of that, eight runners could end up on base and the ninth could hit a 9-run home run. Unfortunately we’ll never get to see that happen… just like we will never see anyone run backwards from second to first base.