I've setup the plugin to fail the build when >=10 new FindBugs issues are detected.
I've got normally around 20 unresolved warnings in my Maven multimodule project.
Now the following can happen:

• build n fails because of compilation error => warnings go down to < 10
• build n+1 compile again, but is set to failed by FindBugs plugin, because it
  thinks there are more then 10 new warnings

My suggestion: the plugin should compare the number of 'new' warnings against
the last successful build. And probably skip the 'new' check altogether if it
cannot find a previous successful build.