Barrie Treloar recommended that I translate some recent confusion into a JIRA. Here goes.
Step 1: run mvn on a project. Get told that the desired version of a dependency was not available. The problem was that a group ID from jetty6 had somehow ended up combined with a version suitable for jetty7.
Step 2: waltz around all the seemingly relevant POMs once, find nothing.
Step 3: run m3 with -X. Aha: a clue. A line stating that the this dependency had been 'managed' from one version to another. Of course, due to -X, this was buried in an avalanche of hard-to-read logging.
Step 4: rummage some more, and find some stale dependency management in a parent POM that applied a property that I now use for jetty7 with the jetty6 groupId.
This problem would have been found in an instant if mvn dependency:tree were annotated with 'management' information: is the version chosen via dependencyManagement, and, if so, FROM WHICH POM.