In the Let's Encrypt staging environment, we are now obtaining SCTs from a new, experimental CT log implementation which includes an SCT extension.
This should be of no consequence to almost anyone, unless they attempt to parse the SCT and don't handle unknown extensions properly. Because there are currently no SCT extensions used in the wild, there are some concerns about compatibility. We are interested in hearing about any incompatibilities encountered.
This change will not be made in production at this time. More information about this new CT log implementation will be made in the coming weeks.