With Steve Kerr coaching the Golden State Warriors, the Cleveland Cavaliers are 0-5 in NBA Finals games where Mike Brown was a head coach.
As head coach of the Cavaliers, Brown led Cleveland to the 2007 NBA Finals. It was the first time in franchise history the Cavs made it to the championship. However, the Cavs were unable to win a game and were swept by the San Antonio Spurs.
Brown was once an assistant coach for the Spurs, so this is not the first time he has coached against a former team for a championship ring. The Cavaliers still send paychecks to Brown. Cleveland will continue to pay him until 2020 even though he was fired in May 2014.
He took over as an assistant coach for the Warriors in 2016 and began his interim gig when Kerr left the game in April 2017. As interim head coach of the Warriors, Brown was aware of the pressure to succeed (via ESPN):
We’ve won  in a row now [this postseason]. But imagine if we were to have lost one? It would have gone from whatever it is now to ‘It’s Mike Brown’s fault.’
He was undefeated as the interim coach of the Warriors, including one win during the 2017 NBA Finals. This makes Cleveland 0-for-5 in NBA Finals games when Brown coached.
Brown was originally expected to coach Game 2 but Kerr instead took the opportunity.
For comparison, Lakers coach Luke Walton was 39-4 as interim coach for Golden State when Kerr was forced to miss time last season as well. If all goes well for Kerr, he will likely remain in the position as the championship series continues.