config not working

hi, when i upload my site i get a page saying:-

 Server Error in '/' Application           

runtime error

then it says to add <customErrors mode="off" /> to the web.config file system.web section which i have done.

but i still get the same message like it isn't taking any affect. is my config file being over ridden or something? what do i need to do to fix this so that i can see the error and debug my site?

 

Thanks,

Duncan
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