In order to prepare for some integration testing of a multi-WAR service layer, I included one WAR into the project dependencies of a Maven web project. If I had time to do this in 'the maven way', I'd probably pull this into an integration test subproject, but, sadly, I don't have time for that in this instance.
Unfortunately, this leads directly to Maven's principle of 'most surprise'. Turns out, if your web project depends on a WAR, the contents of that WAR will be overlaid over your web project, unless you explicitly tell it otherwise. Since I hadn't read about overlays this was actively damaging to my project, and very confusing. This cryptic build output helped me understand what was going on:
OverlayPackagingTask performPackaging overlay.getTargetPath() null
I hope that my mentioning this will save some of you future grief.