Marking several messages with Shift+Down (or Up), and then (without releasing Shift) using the Shift + mousebutton to select other messages doesn't remove the previous selection. The first selection doesn't get removed (Not even when selecting other messages). This seem however only to be visibly, and doesn't affect what is selected. The selection is removed if going to another folder, and back.
*** This bug has been marked as a duplicate of bug 3321 ***