I'm trying to use a file in my templates folder to create a custom 404 handler, but whenever the 404 triggers it's raising an Access Violation. 
Any ideas as to what could cause this behaviour?

Hard to say without seeing the handler code, but I would first check the template path and whether the file is being loaded after the response object is already in an error state. If you can post the minimal 404 handler code, it should be easier to spot where the access violation is coming from.