eXist 1.2.1 is mainly a bug fix release, which addresses a number of stability- and performance-critical issues. All releases in the 1.2 series are considered to be stable. They are limited to hand-selected changes, which have been ported from the current development trunk. New features or major code changes will be part of the 1.3 development series.
We nevertheless count more than 60 bug fixes in 1.2.1!
In particular, an error in the computation of node ids caused database corruptions after repeatedly removing, then re-inserting nodes into a document tree. This bug has been around for a while. Applications which rely on XUpdate or XQuery update extensions should be updated to the new version!
We also fixed a few performance issues, including problems introduced by the new XQuery optimizer. Concerning concurrency, a bug in the locking code could considerably slow down eXist in a multi-user environment, and another issue led to the infamous and usually fatal "document id and proxy id differ" errors, which were reported by users. 1.2.1 also features a new consistency check tool, which should help to detect and fix errors earlier.
A detailed change log is available: