dev-java/prefuse-20060715_beta (main tree) depends on lucene-1* and uses jars from lucene-1. While this works fine with lucene-1.4.3 (also from the main tree), the lucene-1.9.1 ebuild from the experimental overlay is slotted 1.9, thus meeting the dependancy, but not the required package slot.
*** Bug 193358 has been marked as a duplicate of this bug. ***
Looks like betelgeuse fixed this in January already.