Troubleshooting Drupal Ad Module

Recently one of our Drupal sites had a surge in traffic and had to be quickly moved from a VPS to a dedicated server. All went well, except that that the banner ads, powered by the Ad module, stopped appearing.

After checking the obvious (image paths, module settings, permissions) and finding no problems, it was pretty clear there was a server configuration issue, not a Drupal issue. But where to look?

Debugging a live site can be tricky. Enabling debug mode was not an option, but a fix was critical. We began looking at the module files when we spotted the error logs. There was the answer: PHP memory limit. The new server should have had the same config settings but it did not. All of the modules worked fine except the Ad module, which exceeded the memory limit and rather gracefully decline to display ads. Next time a module fails, check the error logs first.