App Engine serving old version intermittently
You have multiple layers of caches beyond memcache, 
Googles edge cache will definitely cache static content especially if you app is referenced by your domain and not appspot.com .  
You will probably need to use some cache busting techniques.  
You can test this by requesting the url that is presenting old content with the same url but appending something like ?x=1 to the url.  
If you then get current content then the edge cache is your problem and therefore the need to use cache busting techniques.
I've deployed a new version which contains just one image replacement. After migrating traffic (100%) to the new version I can see that only this version now has active instances. However 2 days later and App engine is still intermittently serving the old image. So I assume the previous version. When I ping the domain I can see that the latest version has one IP address and the old version has another.
My question is how do I force App Engine to only server the new version? I'm not using traffic splitting either.
Any help would be much appreciated
Regards,
Danny