"aspnet_merge.exe" exited with code 1

"aspnet_merge.exe" exited with code 1 is a nasty build error.

You can see an example and a tip on it at
http://geekswithblogs.net/michelotti/archive/2006/07/31/86754.aspx.

But in my case verbosity is not helping.
The trick was that two aspx files existed with the exact same name (it's not a simple project/solution, we have a framework with files aspx and ascx in it and a custom project with files in it, and the build process builds them together).