Not only 404 but also 5xx errors. As an example, I run an online store, Google checks the landing page urls for ads once a day. If you happen to have a tempora

Not only 404 but also 5xx errors. As an example, I run an online store, Google checks the landing page urls for ads once a day. If you happen to have a temporary outage (surprisingly common on Heroku) at that exact moment your ads will be deactivated and not run for up to 24hrs! I’m somewhat suspicious it also dumps your ad back into a learning phase.