Critical Criticals:

In The Unlit World, it's possible to truly score a critical. A critical critical, to put it that way. If one rolls within the weapon's threat range, and does so again on the check to resolve the threat for a critical, you have a threat for a critical critical. On any secondary checks, only that weapon's 'natural' threat range plus any modifications for Feats are counted - the keen quality or the vorpal quality doesn't count. Succeeding at a critical critical, increases the critical multiplier by one.

There is no upper limit on a critical critical - technically, one might score arbitrarily high, provided you just keep rolling high enough. So far, in our campaign, I believe the highest multiplier scored was x7, something which did an incredible amount of damage.

Fumbling Fumbles:

Correspondingly, as it is possible to score critical criticals, one might also mess up to an extreme amount. To use the parlance from criticals, if one rolls a natural '1', one 'threats' on a critical fumble. Rolling a '1' again, means it will be a fairly annoying fumble. You are also then threatening on an even more extreme fumble. Rolling a '1' multiple times in a row, ensures that the DM will pause for a moment, chuckle evilly, and then adjucate a horrible result for the incredibly bad die roll.

Possible results, from the less severe to the more severe, might be that one accidentally drops the weapon your shield, that you drop your weapon and fall prone, that not only did you drop your weapon and fall prone, you also dropped your weapon in such a fashion that it skitters away a few feet, and, obviously, for an incredibly bad roll, an outcome might be that you slip and fall prone, unwillingly throwing your weapon in the direction of one of your comrades as well as breaking a few breakables in your pack that is suddenly squished under your weight when you fall on your back.

Magic

Shield and Magic Missile:

As the description of the shield spell clearly states, it is an effective defense against the magic missile spell. This is obviously true for The Unlit World as well. However, it is not defined whether the shield spell merely negates any magic missile spells that happens to hit the shield, or whether the shield spell and the magic missile spell cancel each other out.

The ruling in The Unlit World is as follows: A magic missile spell will cancel a shield spell - but the shield spell will remain active until the end of the round that it was hit by a magic missile spell. This does mean that one shield spell might be active against any number of magic missile-like effects, provided that all of the missile effects hit the shield in the same round.