Bug#684153: puppetmaster must be upgraded before upgrading puppet
If you upgrade a puppet managed system from squeeze to wheezy and your puppetmaster runs squeeze, then you get this error message:
Could not retrieve catalog from remote server: Error 400 on SERVER: No support for http method POST
This message obviously tells you, that you are running a 2.7 agent against a 2.6 master. After this message puppet will refuse to function at all until you upgrade your master. Some sort of warning for the uninitiated would be good.
I tried to write a patch, but I was unable to find a development snapshot of the upcoming release notes.
 Well not that obvious. http://bitcube.co.uk/content/puppet-errors-explained