This one is a little tough to describe, and I don't really have a "clean" repro, but please have a look at this image:

  http://img215.imageshack.us/i/wrongmessageinpreview.png/

Notice that the highlighted message is from "Chandan.Rao...", but the message displayed in the preview window is from "John Woodlock".

The index/preview discontinuity seems to occur like so:
* open Evolution after booting computer
* try to read a new message in gmane.come.finance.mifos.devel
* Evolution displays a message similar to "cannot download message from server"
* from then on, all other new messages shown in the preview window don't match the message highlighted in the index

I marked it "major" because I don't know of a workaround, and this blocks me from replying directly to messages on this newsgroup via Evolution.

I'm using Evolution 2.26.1 on Ubuntu 9.04.