Notice (2018-05-24): bugzilla.xamarin.com is now in
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Very commonly the running of the application in the simulator results in this generic error message:
Starting iOS simulator 7.0
ERR: There was an error launching the application: iOS Simulator failed to install the application.
The developer has no way of knowing what the problem is and has to resort to commenting the last lines of code with trial and error until the application is able to launch in the simulator again. The application should not build OR the output should show a more descriptive error.
I have not been able to pin point the cause of this behaviour although it happens quite oftenly. I am able to overcome it though by exiting the simulator application so that next time I run/debug the project the simulator is launched again.
The simulator support included in Xcode 5 (and used by Xamarin Studio) is known to be fragile . Some of the answers in  might help you minimize them.
Hopefully there will be updates from Apple. If you can pinpoint some cases please let us (or directly Apple) know about them (we can file issues with Apple).
Seems similar to this issue: https://bugzilla.xamarin.com/show_bug.cgi?id=15444
We have not received the requested information. If you are still experiencing this issue please provide all the requested information and re-open the bug report. Thanks!