It's never been done, but here are the times it was pushed to 6 or 7 games: http://www.nba.com/news/extendingseriestogame6.html It has been done in other major North American sports 5 times: http://newyork.cbslocal.com/2015/10/21/mets-cubs-nlcs-yankees-red-sox/ The most famous is perhaps the Red Sox v. Yankees 2004 ALCS. That's the only non-hockey one. I think it's likely most common in hockey because the games are the most variable of the major four North American sports. It's low-scoring, so each goal is highly significant. One goal is potentially enough to win a game if your goalie is playing amazingly. Alternatively, if their goalie is playing worse, that's potentially enough to switch the series. There's more too it, but I still think it's the most variable of the four, and this illustrates a piece of it. I think baseball is probably the second most variable when considering similar aspects. Basketball is very high scoring and the goals individually aren't nearly as significant. The game isn't as variable, and even if there is one game where one team shoots lights out and/or the others don't play up to it, it's much more unlikely to happen 4 times in a row. I still have faith though! If our whole team steps up and plays like they should all four games, we have a solid chance.