(110 H 34 M 66779X): Game-Time: 11:49 PM. It is night.
Real-Time: Sun Apr 29 18:33:08 2007 (Central).
Time left on current pray: 42 seconds.
Time left on current blood sacrifice: 45 seconds.
Time left before next renounce: 0 seconds.
Time left before next harm touch: 0 seconds.
(110 H 34 M 66779X): Cujo casts a vigor spell on himself.
(110 H 34 M 66779X): ban ban
You regain -6 hit points.
(113 H 39 M 66779X): ban ban

So.. when Bloodsac is on and u ban when your above 100 % health, it subtracts the extra hp.

It's a bug, it'll be fixed soon.

Another thing with death knights, if you have bloodsac going and a DM casts restore, if you have say 200/160 hp, you will be brought back down to 160. If something can be done about this that would be nice, but it's not a really a very pressing issue at the moment.