Umpire

The World Series has just started, so let's look at the word "umpire".

In medieval France, a nonper (literally “not a pair”) was an impartial third party. We borrowed this as noumpere, but after about a century, people started to think that “a noumpere” (an arbitrator in a dispute) was “an oumpere”. The same n-migration phenomenon happened with “apron” (originally “napron”). Oumpere gradually morphed into “umpire” and started being used in sports in the 1700s.