Strangely, this doesn't cause what I would normally expect from a die, which when hit from the browser is a 500 error. It returns a page as normal...

Not knowing exactly where you're ending up getting the error, it's hard to say, but if you've already started sending output to the client, they will never see the error. In cases where it's important that they see the error, you probably want to hold off on sending any output until the very end of the script. [If this is what is happening, when you view source, you might actually end up seeing the error somewhere hidden at the bottom.]