Jumping onto the bad guy's horse and knocking him off is awesome; missing, hitting the ground, and getting trampled isn't. On the other hand, halfassing it, catching one hand in the saddle, and being dragged while you scramble up the horse is best of all.So writes Mike 'Old Geezer' Mornard over at Big Purple.
Part of the referee's role in most trad roleplaying games is to interpret the rolls of the dice in actual play. Often, particularly during combat, the interpretations are pretty simple, yes/no affairs: do I hit? do I parry? Other circumstances, such as some types of skill checks, may lend themselves to more nuanced interpretations, however: if I fail my attempt to seduce the princess, does she simply spurn my advances or does she have me thrown in the moat by the guards?
Referees are often advised, as in OG's example above, to make the results interesting and exciting, or at least not-boring. The approach of 'yes, but . . .' and 'yes, and . . . ' suggests adding complexity to the situation as an alternative to failure. Another approach is to use degrees of success or failure.
Flashing Blades combat rules include something like this. To hit, the player must roll 1D20 under a target number based on their character's attributes and expertise; if the player rolls less than half the target number, then the wound is serious and causes an additional 1D6 damage. A roll of 1 always hits, and bypasses any protection from armor the target of the attack may have. A roll of 20, on the other hand, is a fumble, and may result in anything from loss of actions to injury.
The rules for non-martial skills in FB state, "A roll equal to or less than the attribute indicates that the skill was used successfully," but I can't think of a single referee with whom I've played who didn't carry over the degrees of success from combat to skill rolls as well. In my own campaign, rolling between the target number and half the target number is simple success, rolling less than half the target number is a more significant success, and rolling a one means achieving Master-level success; conversely, rolling between the target number and half the range to twenty is a simple failure, rolling between half the range to twenty and twenty incurs some consequence for failing, and twenty is complete failure, likely with a dangerous consequence as well.
To use OG's example of attempting to jump onto a horse and knock off the rider, an Acrobatics roll is required to leap up behind the saddle. Let's say the character needs a fifteen or less to succeed: on a roll of 9-15, our swashbuckler manages to mount the horse behind the rider, and in the next round a successful grappling roll is required to knock the other rider off; on a roll of 2-8, I'd allow the grappling roll in the same round, and on a 1, the rider must make an immediate Dexterity check or tumble off on his own. On a roll of 16-18, however, our erstwhile swashbuckler is unable to gain a purchase on the rider or saddle and simply slides off the horse on the opposite side - make failure interesting, or at least a bit comical, remember? - on a roll of 19, he slides over the horse and catches his lace cuff on the saddle, trapping his arm until a succesful Dexterity roll works it loose, and on a twenty, the swashbuckler does indeed end up under the horse's hooves for an immediate trampling attack, if the rider should wish it - hey, it ain't all pratfalls. All of these results are likely to incur a Horsemanship check at some point as well.
There are circumstances in which outright failure is indeed an option, but playing about with the margins can be much more fun, both for the referee and the players. One of the most important aspects of refereeing, in my experience, is training my mind to improvise exactly these sorts of results quickly. I've found there's no substitute for being well-versed in genre tropes when it comes to making these calls in actual play, and that's the main reason I devote so many blog posts to books and movies and art; Cinematic and Wednesday Wyeth and The Pen and the Sword aren't filler - they're my conditioning program for the heavy-lifting of refereeing.