I’m posting this in response to the enormous amounts of times I see GMs and players treating critical successes and failures incorrectly. Now granted you can play your game in your own way, but that does not mean you aren’t doing it incorrectly.
A Critical Success does not Guarantee Success.
This is the one I see the most often. All too often a natural 20 in Dungeons and Dragons is considered an “automatic success”, no matter what the character is doing. And not only do both players and GMs consider it an automatic success, they consider it a success far beyond the ordinary. This is especially wrong when someone who is not well trained in the task makes the roll. While that is great for a moment of entertainment, in the end I feel it just comes down to bad storytelling on the GMs part.
There is a story my friends and I tell sometimes, about a GM we had that allowed us so summon a sailing ship above several unexpecting mooks. The mage that did the summoning rolled a natural 20 on his stealth (resulting in 22) and because it was a natural 20 the GM said they were completely unaware of what he was doing. Meanwhile, the rogue rolled a 13 (totally 22) and he was spotted and nearly killed. The end result of the roll was ignored and the dice result is all that mattered. When we tell this story, while it is entertaining in its own light, I feel the focus is more on how the GM allowed us to do something that he really should not have. Just because we got lucky, and in the case of D&D there is a 5% chance of thus luck, we are rewarded exorbitantly.
A Critical Success guarantees progress. It means you did well despite the odds. The results are on the dice and if you want to make that 5% chance mean something more, make it +5 or even +10. I believe 3rd edition Dungeons and Dragons even recommends just that. You don’t automatically unlock The Vault of Secrets with a toothpick because you rolled 20, but maybe you realize there is another way in. You don’t automatically seduce the barmaid just because you rolled 20, but maybe she’s willing to give you a chance or someone else likes your style. You don’t convince the merchant to give you the Armor of Invincibility just because you rolled 20, but maybe he’ll give you a discount or accept some trade. Maybe even send you on a quest.
A Critical Failure does not Guarantee Failure.
In my opinion, this is even worse, especially if you are dealing with a very skilled character. The expert, specialized level 10 rogue is sneaking through the castle keep. He has +23 to stealth, rolls a 1 and suddenly is surrounded by guards. If you are skilled enough at doing a task and the task you are doing is not particularly complicated, a critical failure just means you did not do as well. You can still slink through the empty halls with no problem. The only reason you are rolling a check in these cases is because the GM doesn’t remember what your skill is.
A critical failure should not be punished more than a regular failure. Fumble charts are the worst. You missed your attack, accidentally threw your sword across the room and stabbed your friend in the side, killing them? That would not ever happen. fumble charts punish skilled characters in ways that are inconceivable. At the very least, fumble charts should also include your actual skill in them. Your skill is literally your ability to compensate for mistakes and unknowns. At worst, A GM should consider a critical failure -5 (for d20s). Players shouldn’t be punished and fail on 5% of their dice rolls.