The Just-World Fallacy is a form of/consequence of the is-ought gap. The idea is that just because something is a certain, that does not mean that it should be that way. So the Just-World Fallacy would be seen in the example "everyone always gets what they deserve." As far as a description of what happens in the real world, this seems obviously false, unless you do a lot of work to redefine what it is that people "deserve".

This is confusing because we use the terms of "should" in a predictive/descriptive sense (if you throw something up in the air, it should fall back down - gravity) as well as a prescriptive/normative sense (you should be kind to your family), as Tycho is talking about.

This implies that those who think they put in the honest effort to get an effect simply didn't actually know what they were doing well enough, which doesn't preclude that a person who actually knows what they're doing can form the actual causes which are guaranteed to produce the effect.

This is true in a sense, but it seems to miss the point. Let's say that I am accused of stealing someone's wallet. I didn't steal their wallet, but I still lose the trial and go to jail. This seems to not be just, but quite possible. Likewise if I had stolen it but not sent to jail, that also seems unjust. It seems that justice has little to do with what I am good or bad at doing so much as what is right and wrong.