My colleague Paul Symes Adam Perry recently discovered a recurring error in the umbraco log,
“Item has already been added. Key in dictionary: ‘RegEx’ Key being added: ‘RegEx'”
sometimes he would also get the yellow screen of death. He managed to track it down to an issue with application start and UrlRewritingNet dll. The issue is detailed here on google UrlRewritingNet forum.
According to the post UrlRewritingNet 2.1 will have the fix, however we are still on 2.0.6 which is used by Umbraco. I have downloaded latest source of UrlRewritingNet and updated the code. You can get updated source and binary here