The Second Life Viewer does not respect the priority of animations with respect to Hand Poses. A Priority 1 pose with hand poses included can override the Hand Poses of a Priority 4 animation, which of course kind of ruins the whole idea of having priorities in the first place.
Steps to Reproduce:
1) Upload any random animation, specifying a hand pose of "Fist". This should make both hands fists.
2) play Animation, and be amazed as the hands randomly change from fist to relaxed over the course of several minutes.
3) stop and play and watch as sometimes it works and sometimes it doesn't. It's the new form of Gambling in SL. Will you get fists, or not! Nobody knows! place your bets now...
You can use the attached animation, upload it with the following settings:
Loop ON, Settings IN: 100% OUT: 100%
Hand Pose: Fist
The other settings can be defaults.
(Note that now I can't even get a fist on upload at all...)