This happens only rarely, but if you consume a buff item (i.e. a favor item) and then IMMEDIATELY click the gathering node (the timing must be just right), your game completely locks up. The character enters the "gathering mode" but no gathering window shows up. You cannot move, change classes etc ("cannot do this while gathering") but you can't use any gathering skills either ("can only do this while gathering") and of course you cannot gather because there is no gathering window visible. The only thing you can do is CTRL+ALT+DELETE and forcefully close the client (because hitting ESC does not bring the menu).

Needless to say, this is quite critical when it happens after consuming a favor buff because at least half of the buff is wasted (or all of it if you are not fast enough with relaunching the game).