Microsoft is pushing their new mobile Operating System as one that won't take all of your time, because you won't have to dig through screens, or other things to find what you're looking for. Simply access the home screen, and you'll find those Live Tiles, letting you know in a glance how many emails you have, missed calls, or text messages waiting for you. That way, with a glance, you can determine if you need to actually start using your phone right then and there, or if you can wait. However, it looks like if you have more than 15 of those Live Tiles positioned on your home screen, you will see an error more than something useful.
This isn't a mistake, and Microsoft is fully aware of it. In fact, it looks like they've designed Windows Phone 7 to work like this. So much so, in fact, that they have official documentation on the "error" that pops up:
"This exception is raised when either trying to open more than one notification channel per application, or when trying to open more than 15 notification channels per device."
This is what the error looks like when you do manage to go over 15 applications with push notification support: "InvalidOperationException(Channel quota exceeded)". It's interesting to see that the error is actually shown as an error, especially when it looks like Microsoft designed the system in this way. There's no telling exactly why the Redmond-based company would want the system designed like this, but the obvious guess would be to make sure that the user's experience isn't muddied in any way. Of course, 15 applications is a lot of applications to be running on the home screen, all with push notification support. As of right now, that may not be an issue, but when third-party applications can start running in the background, it will be interesting to see how many people start seeing this error.