Symptoms

A stacktrace and a long series of unhappy messages in the logfile. A dialogue box with the following contents:

The plugin version I expected is not what I found

Plugin Key: <plugin name>
Expected: <version number>
Found: <different version number>

Are you sure you would like to continue installing?

Cause

Plugin author forgot to update <version> within atlassian-plugin.xml.

Resolution

Notify the plugin author, and explain that they need to fix atlassian-plugin.xml for that version of that plugin.