On a Joomla site running Gantry 5 template I noticed my custom 404 page stopped working. I looked up the process and when I looked at the files on the server I had an custom error page there already; however, the date on the file was in October and I put the custom file up there in March.
I was wondering if I got attacked and my files were changed. I didn’t notice anything other issues, but several files were changed in the g5_hydrogen folder. I then realized that around that time I upgraded my Gantry Template and Hydrogen Theme around October.
My conclusion is if you update the Gantry Hydrogen template, then make sure your custom 404 page is still working. I haven’t tested this in the WordPress version, but I would assume it is the same.
I hope this helps someone else out there…