There appear to be updates to the label manager interface in 4.2-beta1.
The changes are not binary compatible with plugins compiled against 4.1 and earlier Confluence versions. Normally we would expect to see a method become deprecated for a few releases before it is removed/has its signature changed.
Example: Adding a label to a page via the call to labelManager.addLabel now throws:
For code compiled against 4.1 and running in 4.2. The same code compiled against 4.2 works fine. Unfortunately compiling all code against 4.2 is not an option due to the new interface EditableLabel not being found prior to 4.2.