Sometimes people update their services and don’t see what they expect after.
With Fastly if you make a configuration change that change is pushed automatically and you should start seeing it within a few seconds. This is the same for custom VCL and changes in the settings tab on a service.
People can get confused when they do’s see their changes in production. To clarify configurations are pushed immediately, but the cache responses that correspond to the service are not purged immediately.
I have seen a few problems recently with updating services that have compression. If you update a service to deliver compressed objects and the cache servers are still delivering uncompressed objects it might be because you didn’t purge. The new version of the object has not gotten into the cache so requests get the old object.
You should update your cache by purging after a configuration change that affects your cached responses.