I just upgraded from Jenkins 1.502 to 1.503. "Discard old builds" has been broken.
When I go to the "Manage Jenkins" page, I see "You have data stored in an older format and/or unreadable data.", and clicking on "Manage" shows "InstantiationError: jenkins.model.BuildDiscarder" for most of my jobs. All my jobs that previously had "discard old builds" specified don’t have it ticked any more.
I presume this is related to this changelog entry: "Discard old build records" behavior is now pluggable, allowing plugins to define custom logic.
If I edit a job and tick "Discard Old Builds", previous values are not there, and there's some spurious text on the page "//x", presumably something that was intended to be removed when this was released.