Milestones
Monotone version 0.26 introduced major changes to the internal database structures, including a new structure known by Monotone developers as a roster. Monotone databases created with version 0.26 can not exchange revisions with older Monotone databases. Older databases must first be upgraded to the new format. The new netsync protocol is incompatible with earlier versions of Monotone.
Read more about this topic: Monotone (software)