If you select a revision with a left-mouse click, then Ctrl+click to select another one, it doesn't really work. The first time you ctrl+click, you will have selected the new one, but deselected the old one. What's worse is that the UI will //not// change when you have done this; it will still look like you have selected just the old one when you have selected the new one.
Furthermore, if you ctrl+click twice, you can make the UI look like you have selected multiple non-consecutive revisions, but in reality you will only have selected one.
Either the UI needs to forbid non-consecutive multiple selection (possibly with a sound when you try to ctrl+click), or the UI needs to make non-consecutive multiple selection work.