1. Install plugin usage plugin, conditional build step plugin, and a third plugin that can be used in the conditional step, for example 'fail the build' plugin
2. Set up a job that uses the third plugin in a conditional build step
3. Plugin Usage Plugin reports conditional-buildstep usage as 1 (OK)
4. Plugin Usage Plugin reports the third plugin usage as 0 (NOT OK)

This is a false negative that can lead to people accidentally removing plugins that are in use.