After some investigation it seems that this is caused by rails ‘marshalling’ the data. If you’re using a memcached set command such as
Cache.set(key, content, time_to_live)
You need to add an extra argument to force rails to cache the raw data instead of marshalling it :
Cache.set(key, content, time_to_live, true)
Restart memcached to make sure the old data is cleared, and the problem should go away.