When I talked about PCs using poisoned weapons, JD suggested that hitting an ally by accident would be a good fumble result. By this, I believe he's referring to the nearly universal interpretation of a 1 on a d20 attack roll as a fumble, paired with a 20 on a d20 as a critical hit. It wouldn't be unreasonable; certainly, I've seen people suggest this for firing arrows into a mêlée. But I have a problematic relationship with fumbles and criticals.
Paradoxically, it's because I find them both too common and not common enough. A 5% chance to fumble, and conversely a 5% chance to automatically hit, seem too high to me. Although hitting an ally or even yourself by accident during a mêlée wasn't unheard of, it seems like it shouldn't happen all that often, and it should be something a player can avoid by making a safe choice instead of a risky choice. But when a player does choose something risky, it seems like the risk ought to be higher than 5%, as an encouragement to consider options instead of always taking the same kind of actions.
So Part One of my solution would be to restrict fumble and critical results to special situations. I mentioned curses in the previous post, precisely for this reason. Aside from some specific cursed items, like cursed scrolls, there's really no description of the kinds of curses that could afflict a character. The usual result is to use a bland "reverse Bless" spell that gives the cursed victim a penalty on attack rolls and saves. It would be far more interesting to say that a generic curse means that failures in the victim's presence are likely to turn into fumbles that harm the victim. So, a cursed fighter using a poisoned sword is likely to injure himself or an ally as a result of the curse.
Part Two of my solution (increase the odds of a fumble showing up in risky situations) is suggested by the purple worm. There's a hidden progenitor of the critical hit in the purple worm description: if a purple worm scores a natural 20 *or* 20% more than is needed to hit (target number +4, in other words,) then the worm swallows a victim whole. I could see using this as the mechanic for critical hits in appropriate situations, instead of just a flat 20. And we could use a similar approach for fumbles: any risky attack roll that is half the minimum needed to hit, or a 1 in any case, is a fumble appropriate to the situation. It makes cursed individuals much more likely to fumble, and we could apply this rule to untrained actions, like a magic-user trying to use a sword in desperation.
In cases where the chance to fumble is high, I could see reserving a natural 1 for an extreme fumble: a catastrophe. This may be pertinent to the poisoned weapon situation, because perhaps an ordinary fumble should be a dropped weapon, with injuring allies only happening in the more extreme cases. At the very least, it gives us an opportunity for a minimum of three failure results (ordinary, fumble, catastrophe,) instead of just one or two, when dealing with risky situations.