Intended as a repository of pointless bits of trivia I or others find, particularly while browsing the database. It happens a lot between basho.
In a total of 5 basho since the 15-day era began, the last match was between two undefeated rikishi. Twice it was Taiho vs. Kashiwado, and they split the matches. All matches featured Y1e. No such match was a playoff (theoretically possible between stable members). Harumafuji is the only one to appear in such a bout that was not a Yokozuna at the time.
No other Day-15 matches involving 2 14-0 rikishi have occurred. In Haru 1996, there was a 6-0 match in Jonidan on Day 15, but two other rikishi were also 7-0 from previous days. Incidentally, a few basho in each direction in that time period there were many Day-14 6-0 matches in the lower divisions, but none that basho and one on Day 15 instead. Only one Day-14 6-0 match took place outside that 1995-1996 window.