There are 6 No. 8 NBA teams that have won over No. 1 seed teams in the history of NBA Playoffs since it was expanded to the current 16-team size in 1984. The first one occurred in 1994 when the Nuggets beat the Super Sonics in 5 games. Miami is on this list twice, once as the losing team and once as the winning team.
1994: Denver Nuggets d. Seattle Super Sonics 3–2
1999: New York Knicks d. Miami Heat 3–2
2007: Golden State Warriors d. Dallas Mavericks 4–2
2011: Memphis Grizzlies d. San Antonio Spurs 4–2
2012: Philadelphia 76ers d. Chicago Bulls 4–2
2023: Miami Heat d. Milwaukee Bucks 4–1
Has an 8th seed ever made the NBA Finals?
Yes the 1998-1999 Knicks not only upset the No 1 seed Heat, they also made it to the NBA Finals but lost to the Spurs in 5 games.