If you sort the mail by sender, then select and move multiple messages to a different folder by drag and drop, when Roundcube moves messages from the second page onto the first page it moves them out of order. Additionally, clicking on the subject of one message (to select it) occassionally selects a different message entirely.
These problems are always reproducible for me on Safari 1.3.1 and OS X 10.2.
Additionally, there is an intermittent bug when the wierd selection condition occurs that none of the interface buttons work. The only thing it is possible to do from within the Roundcube interface is to open the message, and then you can refresh the client by clicking on the logo - but no other "clickable" areas work. I can reproduce this one occasionally, but it isn't reliable.
Stephen