There’s a maximum of 255 local messaging channels in Silverlight 4
Don't ask me how I got into this situation, but I found this out the hard way. A quick search will tell you that with Silverlight Local Messaging each message has to be under forty thousand characters, but I haven't found anything on the internet yet that tells you that you can have a maximum of 255 channels open at a time. So I'm taking responsibility and putting it out there. Going over the limit will give you a cryptic COM exception, and channels can be "leaked" if one browser process crashes while another is open (for example iexplore.exe and chrome.exe, or just two instances of chrome.exe). Often closing all browsers will clean up the phantom channels, but not always. Remember that other Silverlight applications might be using some of these channels too, so you should always design your application such that it uses a minimum number of channels.