When the Cobertura plugin is configured to fail a build, or show a build as unstable for dipping below coverage thresholds, there is no obvious message on the build page that shows this. The user sees there are no test failures, and coverage percentages for Packages, Files, Classes, etc...
If the user is not familiar with what the configured thresholds are, they may not know that a Lines percent coverage of 82% is under the threshold of 90%, and that's why the build failed. This is particularly important when using auto-update thresholds. While the information is in the build log, logs can be long and complicated. A message on the build page would clear up this confusion quickly.