WHMCS Smarty {include} Not Working

I’ve been using Smarty templates a bit recently while working with WHMCS and ran into an issue. When trying to do {include 'some_template.tpl'}, I was getting no output. Smarty seemed to just stop all processing at that point. The fix was maddeningly simple, especially as I’d tried a bunch of different variations of absolute and relative paths. If your template is in a file called some_template.tpl, you need to use:

WHMCS 500 Error

While working on an addon module for WHMCS I kept getting a generic 500 error code and a blank page. I was pretty sure it was a problem in my code but there was nothing in the XAMPP error.log or the WHMCS admin dashboards so I had no way of tracking down the cause. I managed to finally track down the cause by enabling the Display Errors option under Setup → General Settings → Other.