Version:            (using KDE 4.2.0)
Installed from:    Ubuntu Packages

Open Kmail.  Create a new, empty folder to be the destination.  Select a folder with messages in it.  Type the letter S or use the menu to open the Find Messages modal dialog window.  Do a search which pulls up a subset of messages in that folder.  Select one or more messages in this dialog window, and drag them to the main Kmail window  and the new empty, folder, which gives you a choice to copy or move them to the new folder.  Do so.  The wrong messages are copied or moved.  The messages which DO end up being copied or moved are "close" in time to the messages which I intended to be copied or moved.

I believe this is related to messages being deleted from the original folder but the folder indexes not being updated; after I select "compact folder" the problem ceases to exist.  (The problem is easily reproducible if you have a test folder, delete some messages in it, and then do as I've described.)  However, the problem should never exist - if you're going to move a message out of a folder based on an index, you should make sure the index is up-to-date beforehand...   The folder I've observed this on has about 6000 messages and is in maildir format.

Interestingly, the problem doesn't exist if you move the messages from the main window, only from the search dialog window.  Moving messages from the search dialog window is definitely screwy - sometimes they don't move at all, nor can several messages be moved at once (even though they can be selected and dragged).  I would guess the search dialog is creating its own temporary index of messages-which-match-the-search, and this index just isn't working correctly.

Expected behavior: moving messages from the search dialog window should move the correct messages in all cases.