This is just a stray idea, I had a month or two ago, which I feel like sharing. As some of you already know, I am highly sensitive to probabilities, and I dislike most RPG roll mechanics because they are (for lack of a better word) "excessively random". The dice weigh in too heavily, compared to the intrinsic competence (or incompetence) of the acting individual.
Back in 1998, or possibly early 1999, I tried to figure out a way to make a Critical Success/Fumble mechanic for the Multiclass RPG, which uses 3d6 for skill rolls. But I could not get things right. Recently the solution occured to me:
If you roll triples (e.g. 111, 222... and so forth) and the roll sum is three or more lower than your skill, then you've gotten a Critical Success. Notice how this means that you need a skill of 21+ to have the highest possible chance of getting a Critical Success: 6/216 or 1/36. And of course it should be effective skill, e.g. the skill level after bonuses and penalties have been applied. Someone with a skill of 10 or 11 (a decent level of proficiency, on any 3d6 based scale) gets a Critical Sucess, on an unmodified roll, only on a 3 or 2+2+2 (6), a 2/216 or 1/108 probability.
The Fumble mechanic is exactly the same. If you roll triples and your roll is three or more higher than your (effective) skill, it's a Fumble. This means that as soon as your effective skill reaches 16, you're immune from Fumbles.
I think this is a very good mechanic, both because the probabilities are decent (they are as good as it's possible to get them, with 3d6), and because it's fast to execute. No re-rolls, and also triples are very quick to spot.
It may seem as if there is a breakpoint problem with this mechanic, but it's actually the other way around. Most of the values, on the 3-21 scale, are breakpoints. In most cases, when you increase your skill level by one point, something ifty happens - either your Fumble probability goes down, or your Critical Success probability goes up. A good illustration of this is to mention the non-breakpoints, those few values where nothing happens.
Here's a list of those values that actually are breakpoints:
03 You can now actually succeeed (1/216 chance)
04 A roll of 6 is no longer a Fumble (4/216 Fumble chance)
06 You can now actually Critically Succeed on a roll of 3 (1/216 chance)
07 A roll of 9 is no longer a Fumble (3/216 Fumble chance)
09 You can now Critically Succeed on a roll of 6 (2/216 chance)
10 A roll of 12 is no longer a Fumble (2/216 Fumble chance)
12 You can now Critically Succeed on a roll of 9 (3/216 chance)
13 A roll of 15 is no longer a Fumble (1/216 Fumble chance)
15 You can now Critically Succeed on a roll of 12 (4/216 chance)
16 You can no longer Fumble (0/216 Fumble chance)
18 You can now Critically Succeed on a roll of 15 (5/216 chance)
21 You can now Critically Succeed on a roll of 18 (6/216 chance)
This variation on the standard 3d6 roll mechanic is obviously of use to people who use (or want to use) the Multiclass RPG, but it can also be used in other 3d6-based skill systems, such as GURPS and Hero.