Our compatibility rules only start to apply once a final x.y.0 version has been blessed and released. Any API's, wire protocols, and on-disk formats that appeared on trunk or in an alpha/beta/rc pre-release are not subject to any compatibility promises; we may change them arbitrarily until the final release, if we find a good reason to do so.
In particular we might not provide an upgrade path for persistent data with the final release. Consequently, pre-releases should never be trusted with any data meant for long-term safe-keeping.
At the same time, we wish to remind the reader that the best time to point out API design bugs is before they are released and set in stone during the initial design and pre-release stages.
(Thanks to the Subversion team for the content we 'borrowed' from the Subversion release guideline)