From mailing-list entry "Verify downloaded jpi-files":
https://groups.google.com/forum/?fromgroups#!topic/jenkinsci-users/2v8csoO0cxE

Jenkins does not seem to verify the integrity of downloaded plugins right after the download has completed. Rather, the verification is only done when attempting to install/upgrade the plugin.

This concequence of this is that corrupted plugin updates will trigger a plugin uninstall instead of upgrade. Any job-configuration related to the accidentally uninstalled plugin is then also deleted, which is pretty serious.

Steps to reproduce:
1: Create a job with a subversion working-copy workspace.
2: Configure plugin manager with invalid PROXY settings, so that non-intranet HTTP-requests returns a HTML error webpage (instead of connection refused).
3: Upgrade the subversion plugin.
4: Jenkins will download a corrupted subversion.jpi file containing HTML content without any error message.
5: Restart Jenkins.
6: Loading of subversion.jpi will fail (error log attached).
7: The subversion plugin will be uninstalled.
8. Subversion-related configuration in all jobs will be deleted!