In the mercurial version of Carpet, the parameter Carpet::poison_value has been removed. This breaks any existing parameter files which use this (I have a large number). To avoid unnecessary pain, I propose that the parameter be reinstated. It can print a deprecation warning, and doesn't have to do anything.
I think we should aim to achieve backwards-compatibility with code and parameter files as much as possible, especially when it is trivial to do so. The parameter can be removed for good in a later release, if it is felt that it clutters up the code too much. I think there was a CarpetLib poison parameter which also changed, but I don't remember what it was. This should apply to that one as well.