I think it's too easy for a #15 to #25 team to fall out of the rankings, to the extent that it stops being noteworthy. To that end, I'm copy/pasting from a thread I saw yesterday about Top 10 teams that have dropped out of the Top 25 recently.
Begin copy/paste:
Past teams that finished unranked:
2018: #4 Wisconsin, #8 Auburn, #9 Miami (Fla.)
2017: #3 Florida St.
2016: #10 Notre Dame
2015: #6 Auburn, #8 USC, #9 Georgia
2014: #4 Oklahoma, #9 South Carolina
2013: #5 Georgia, #10 Florida
2012: #1 USC, #10 Arkansas
2011: #8 Texas A&M
2010: #4 Florida, #5 Texas, #9 Iowa
2009: #3 Oklahoma, #9 Oklahoma St.
2008: #7 LSU, #9 Clemson, #10 Auburn
2007: #10 Louisville
(Last time it happened to U-M was 2005.)