GMs don't dismember characters...
It figures... out of three alien predators with shoulder mounted blasters, all three catastrophically fumble. The last of the three had a toy the first two didn't... a monofilament whip that might discourage players from going into melee (where most of the players are the most effective).
It did 2d6, with no Strength bonus to damage, which was a bit of a sacrifice considering this alien predator had a +5 bonus from its Strength and an additional +2 due to class abilities. But every hit forced a Fortitude Save DC15. Nothing makes players sweat like melee weapons that force saves. If the save failed, the target took double damage (4d6). If the attack provoked a MAS (massive damamge, greater than the character's Constitution) check, the the save for that was a fail, the character lost a limb.
Even after that, the group's doctor had a better than even chance of reattaching the limb sufficiently to keep it viable until they reached a proper facility (this is the Firefly 'verse, where medical technology is sufficient to replace someone's internal organs with engineered designer organs with no ill effects ["The Message"]).
One PC and one NPC currently need help to applaud. The doctor failed on both of them, so now I may have to actually flesh out cybernetic rules. I'd hate to see what would have happened if my players hadn't played as smart as they did, given how treacherous the dice were being that evening.
The moral of the story: don't rely on the dice. Unlike a smiling GM, they give no warning.