What is this error, and why does it happen?
05-17 18:24:57.069: ERROR/WindowManager(18850): Activity com.mypkg.myP has leaked window com.android.internal.pol
In my case, the reason was that I forgot to include a permission in the Android manifest file.
How did I find out? Well, just like @Bobby says in a comment beneath the accepted answer, just scroll further up to your logs and you'll see the first reason or event that really threw the Exception. Apparently, the message "Activity has leaked window that was originally added" is only an Exception that resulted from whatever the first Exception is.