On the www.wikipedia.org portal, the behaviour of the search suggestions box is different than that of the suggestions boxes elsewhere.
If a user begins to type something into the search box, and then highlights an option with the mouse (either after pressing up/down to go through options or not), and then presses up/down on the keyboard, the highlight does not move to above/below the highlighted option. Instead, it ignores all highlight changes done by the mouse.
So, for example, if a user were to highlight the third listed page, and press up on their keyboard, the selection would move to the last listing, instead of the second listing.
Note: I've verified this bug exists on the wikipedia.org portal, but a similar behavior is noticable on the en.wikipedia.org search box as well - the mouse curser moves immediately to the front of the text in the search box then jumps to the end of the text in the search box. Not a very elegant solution, but it does seem to work better and is more intuitive to the user.