Based on the SAC cases, it seems to me that there is a significant number of users who experience issues with JIRA when they install plugins and the plugin is incompatible.

There are a few common case scenarios for that:

1. New version of the plugin is released with a funky feature and user want to take advantage of it. Or, install plugin for the first time and picks up the latest version.
2. User go to a plugin homepage and sees that more recent version of the plugin was released and believes that plugis are backward compatible.
   These two can be addressed by better indication of this problem on the plugin homepages. - Big red banner saying "Think twice before you select version to download!"
3. User performs the upgrade of JIRA, but didn't follow the instruction from the upgrade page - perhaps we should have a link to the upgrade page on the JIRA download page. Additionally, emphasise that issue in the upgrade page at http://www.atlassian.com/software/jira/docs/latest/upgrading.html