1. Enter a MP game with debug console enabled.
2. Create ten custom radio channels using this:

for "_a" from 1 to 10 do {

_index = radioChannelCreate [[1, 0, 0, 1], format ["Custom Channel %1", _a], "%UNIT_NAME", [player]];

};

3. Try to select any channel using setCurrentChannel N, where N is any number from 0 to 15 (6 default channels + 10 custom ones).

The command fails for 15 (Custom Channel 10) and returns false as if the channel does not exist, despite working with every other custom channel.
"Custom Channel 10" can still be selected using "." or ",", which leads to this scenario: http://i.imgur.com/c5ePolh.jpg

If trying to reproduce this on a dedicated server, then keep in mind that radioChannelCreate has to be executed on the server.