It seems that errors in plugins etc. can cause jobs to not load during startup, without any warning being displayed in the Jenkins UI: you just get a blank screen as if there are no job configurations at all, which is rather alarming. (There would be a warning in the logs, but due to the ring buffer this could be out of view.) Example stack trace:

... jenkins.InitReactorRunner$1 onTaskFailed
SEVERE: Failed Loading job ...
java.lang.NoClassDefFoundError: Could not initialize class ...
    at ...JobProperty.getJobActions(...)
    at hudson.model.AbstractProject.createTransientActions(AbstractProject.java:665)
    at hudson.model.Project.createTransientActions(Project.java:208)
    at hudson.model.AbstractProject.updateTransientActions(AbstractProject.java:658)
    at hudson.model.AbstractProject.onLoad(AbstractProject.java:292)
    at hudson.model.Project.onLoad(Project.java:88)
    at hudson.model.Items.load(Items.java:209)

Jenkins should

1. Catch more errors in Items.load, so that problems might be contained to one job while allowing others to load.
2. Produce an administrative warning that something is seriously wrong (i.e. more than OldDataMonitor can handle), with the original stack trace.

Should be reusable enough that the CloudBees Folders plugin could in the future also isolate problems in individual jobs within one folder.