While most games, including USR, structure combat around an attacker and a defender, I've taken the plunge with simultaneous action in combat. This means any critical hit or fumble mechanic which catches my fancy will have to be modified to account for any one of the combat participants receiving an extraordinary result based on both participants roll.
Also, each actor in the drama does not necessarily use the same dice so I have to consider how to determine critical and fumble results which account for this variable as well. For the raw mechanic I considered how Chaosium's Elric (as well as their BRP system) handled criticals and fumbles. But I don't have the luxury of a generous d100 point spread to move around in and I was concerned that, with combat dice ranging from d4 to d10, a natural 1 is going to come up more often than I or my players are going to want.
Here is how I structured the mechanic; If you roll a natural 1 or natural high #, you compare against opponents roll. If your opponent has also rolled a natural 1 or natural high # it is time to check for Critical Hits and Dramatic Fumbles.
Here is the link to the Crypt Keeper's Screen where the current Critical Hit and Fumble Tables can be found.
All my rules for the game so far can be found on the Summoning page of my blog.