About six months ago we had an issue with incorrect query results. (http://jesseorosz.spaces.live.com/blog/cns!E322FD91218E57CF!250.entry). I had included in an aggregation a reference dimension with materialized set to False and SSAS didn’t like this.
Well much to my chagrin I tried the same thing yesterday in Aggregation Manager and received this error:
Be warned, you’ll have to download the latest version of BIDS to get the newest Aggregation Manager. It’s really been cleaned up since the first version shipped with SP2.