I was thinking about the post about a fumbles by Bias and got into a reserch on the old system made by Issaries for Rune Quest that is D100 based as I thought it would be easier to approach as a starting point to SD's system.
The way I found would require change to the objects to support a fumble and critical description tables.
The way to implement this is comparing the skills to a 0-100% chance.
A special check would be if the 'roll' is under 1/5th of your skill rate. rolling from 1-10% if you have 50% or are in the middle of the skill learning levels. It would lead into doing things you cant do with your actual skill level (Spot someone in a stealth cammo suit, sing a song in Japanese, lift a car from top of that baby and stuff...)
A critical check would be the doing of the impossible, kick thru the doors of the terra offices and sutch, but it would be 1/20th of you skill like a 1-2% if you have the same 50% from above.
Fumbles would work the inverse way criticals do. If you have a 25% skill, your chance to miss is 75% and that leads to a 4% chance of critical or a 96-100% roll. It would cause harm to the player, loss of a round of fight (stun), temporary penalty to all his/hers rolls.
In that way it will be balanced as people with highter ue expended into skills will have a greater margin of critical and specials. As the person that has little or no ue on a skill will be more likely to fumble (A terrible driver hit a tree).
If a Special occurs then it just happens as it normally does to someone that has the correct level of that skill. But on the Critical/Fumbles a small list of 5-10 descs and results could be randomly picked to give the sobranatural or catastrophical result of the rolls would be required for every item.
Or even yet a bigger and unique list of fumbles and another of critical could be used by pointing at a range of them. Like items 1-50 are combat fumbles and criticals, 51-55 Driving and etc...