I'm using the 14.1.5 EAP, but this may apply to the 15 EAP, too - I have not checked.
If I try to install a version of the Scala plugin from a .zip file on disk and the version of the plugin is too high to supported by that version of IDEA, then it correctly reports "Plugin 'Scala' is incompatible with this installation". Screen shot attached.
But if I then try to install an earlier version of the plugin which is compatible, the the same error message is produced each time, this is wrong of course. In reder to install the version of the pluging I have to restart IDEA. This bug can cause frustration, of course.