Yahoo Mail hijacked the down arrow: Looking in the preview pain, press down arrow to scroll... and you jump to a new mail. Very, very jarring and frustrating. I recall they also hijacked a bunch of other keys, none of which made much sense, which is sad... mostly because it shows that they don't use their own products enough to see how this can affect users.
Google+ appears to do the same thing with C-PgUp and C-PgDown; if I had wanted those keys to stop working properly, I would have used some ass-backwards browser like IE. If I've got a Google+ post open and I'm scrolling brisking through tabs, it's like hitting a brick wall.
