Running FindBugs on log4j sources -- as part of preparing lecture, in fact -- revealed a bug in org.apache.log4j.lf5.viewer.categoryexplorer.CategoryPath. In line 64 there is a code processedCategory.replace('/', '.'); and FindBugs correctly recognizes the error there as the result of replacement is ignored. The line should be processedCategory = processedCategory.replace('/', '.');
LogFactor5 is long deprecated and is no longer maintained. Committed change in rev 734230.