I think the best bug was the movement possibilities during spell casting from breaking animations. It created one of the most complex and amazing PK (PvP) dynamics of any MMO to exist.

The complexity of being able to move only so much to still get your cast off, and being able to slightly fast-cast or hold long delay-casts to "outplay" your opponents created so much depth to duels it was incredible.

Even after all these years I can still remember the Arc cast I would do, it was the keyboard combo: Hold Left -> Hold Z -> X -> tap/hold up to control the radius Then Hold Right -> Hold C -> X -> tap up to reverse the arc to return to where you initiated the cast so the spell could go off.

Man I miss this game, I hope someone creates a wonderfully buggy remake someday!