Whoops, our bad…
The page you requested was not found, and we have a fine guess why. If you typed the URL directly, please make sure the spelling is correct. If you clicked on a link to get here, the link is outdated.
What can you do?
Well, you can start by looking in /var/report/ and view the file that corresponds with the error ID on the web address. 
This is just a merely nice page to display to end users as not to display any nasty php errors to them. However looking at these reports can lead useful if your needing to find out more details on whats causing the issues.
Didn’t realize this until tonight, I must be slow… 
but thought I would share.
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B00MER, thank you for sharing your thoughts!
Concerning this post – I didn’t understand – I will receive “Whoops, our bad…” when enter wrong URL, but Error report will appear if something wrong with my backend – not with URL. Or I’ve missed something?
Nuke, Sorry if my posting was not very clear. If you are simply receiving a 404 message no error report is generated. This is merely if there are any exception errors. Hope this helps?