Weird error in log when using Thin Server with a pretty simple rails application

I am pretty new at using the Thin server on my simple rails application. It's working nicely except, that 1 out of 4 times more or less when I go to any page in the app I get this on the browser:

    Status: 500 Internal Server Error Content-Type: text/html     Application error

    Change this error message for exceptions thrown outside of an action (like in Dispatcher setups or broken Ruby code) in public/500.html

and this in the log:

Before I continue to kill myself troubleshooting this, I wonder if anyone has seen this and can help me with a clue or two? Thanks!!