I’ve found a problem when I have a Biped and another object that I want it to pick up. I give the object a Link Constraint, which is initially set linked to the world, at some frame I add a link to the Biped’s hand, then when I move the biped’s hand after that frame only the children of the hand (the finger bones but not the linked object) move, then if I hit the Set Key in the Key Info rollout, it all snaps back to where it originally was.
The strange thing is that although the arm and hand bones don’t move, if there’s a skin on the biped, the skin moves as it should, as if the arm and hand bones were moving properly.
It works OK if you have Auto Key on, but I tend to use Set Key when animating bipeds.
If you’ve got the time, try this out.
I remember seeing similar problems when I first got Max 09 and loaded a character rig I did in Max 08, it was based on a standard biped but I put more bones around the shoulders which had position constraints to point helpers attached to the biped bones.