I've experienced this bug in RC1 too ... I thought it would be solved but still getting this on RC2 ... so I thought reporting this before the RTM goes out.
Also reported by others here : http://forums.asp.net/p/1632696/4214831.aspx
This happens when the application is starting up after some changes to code and possibly after a re-compile happens ...
Please check this bug.
I can't replicate this because it happens in random moments ... but more frequently than you think.