There are some challenge bugs that gets the challenge command bugged (that nobody can challenge again until server reset)

1. When both players leave
2. When you fight with grenade's

i challenged JDizzle once, with a rockit launcher, he kinda shot himself dead and nothing else happened
he died
i stayed in the boxing ring(square)
i did /leave
he did, too
nuthin happened
couldnt challenge anyone after that

you need to do /kill

i did, forgot to mention that.
jdizzle did it too

It happens a lot actually, and even if the players use /leave it still tells the challenger that there is a fight going on at the moment.