Here's an odd one...

Spirit servants sometimes just disappear when you enter the character manager by CHECKing IN. (I know, who does that anymore?) It seems to happen at random -- for me, usually happens, but not all the time. If the servant had items, you'll find them on the ground when you exit the character manager. Spirit servant will still show up under your active spells, however. Seems more likely to happen if you spend more time in the character manager, but I couldn't find any clear pattern (change in fame/exp seems to have nothing to do with it).

Also saw that this bug could be abused -- it miscalculates duration after your servant has been "disappeared" in this manner. Casting 218 summons a new spirit, but adds your base duration to any remaining duration you had on your previous spirit. So it's possible to stack your spirit up to 4 hours by entering/leaving the character manager, casting 218 if your spirit has disappeared, rinse and repeat. Because this is a mechanical benefit (however weak), I guess I should add the disclaimer -- don't try this at home.