PHP Code:
<customErrors mode="On" defaultRedirect="Error.htm" >
</customErrors> 
My site is running in IIS 7 and sometimes after an error is thrown I actually see my error page but other times I get the typical asp.net YSOD. Even though the address to the page is actually showing the same address in the address bar as if the correct error page is actually visible.

Code:
http://site/Error.htm?aspxerrorpath=/site/default.aspx
There is no rhyme or reason to the frequency of this happening. I can restart the web site and it doesn't seem to be affected. Once the correct error page is shown, it will keep being shown for quite a while. Then randomly the YSOD will be shown two to three times until finally the correct one is shown again.