This is kind of weird, but it only happens for me with the Evolution
compser, and only in Metacity.

If you're replying to a large message you will frequently want to select,
say, the last hundred lines of it and just delete them. To do so, start
selecting and then move the mouse outside the border of the window.  This
is where I encounter two bugs:

a) the selection will move to the bottom, jump up about five or ten
percent, scroll quickly to the bottom, jump back up, and go on and on
endlessly like this.  This sort of behavior sometimes occurs in text entry
boxes as well, but I can't seem to duplicate it consistently.

b) When you let go of the mouse button and re-enter the window, the
selection is greyed out and you cannot interact with it. Exiting and
entering the window has no effect. (contrast with any other app, where the
selection will be greyed out when you are outside the window, but return to
active when you re-enter it, so that you can interact with it.)