Configure a job using conditional buildstep plugin.
Choose a run condition, one that can fail.
Choose option "On evaluation failure Mark build unstable".
Make the evaluation of the condition to fail.
Start the job.
After finishing the job, the buildstep is not runned, as expected, but the build is not marked as unstable.
The same happens if "Mark build as failure" option is selected.The build is always marked as succes without taking into account the value of this option.