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: