There's something else with scrimmages...
Opening a team's overview site when I already did 15 challenges displays correctly that I cannot do one more. If I did less than 15 challenges - let's say 14 - and open more than one team overview site at a time in different tabs I challenge the first which tells me correctly that I issued a challenge. Now my list of 15 possible challenges is full but when challenging another team whose site I opened before that limit the site still tells me that I issued a challenge.
It's purely presentational as no additional 16th challenge is done.