1. Open a job configuration that has a conditional build step defined in it, with the Chrome browser (Firefox does not show this problem).
2. Click the back button to navigate away from the config.
3. Click forward to go back to the config.
4. The conditional build step settings will be blank/default, but all other job configuration remains correct.
This is a really nasty bug, I've had a number of users accidentally break their configs without realizing it, because if you save after this happens you lose the conditional build step settings. Luckily the Job Config History plugin makes it easy for us to undo this.