So my knight and my paladin both have a large shield +1, and a large shield +2 respectively, and when I try to remove either of them, it says the item is cursed and won't let me remove it. What exactly does a cursed item like that do? I tried looking it up in the clue book and it never mentions that these 2 items are cursed.

Well, they cannot be removed, so you cannot equip a better item and they do not give the damage and/or AC that one would expect. There are some items such as those you have mentioned that have non-cursed versions and there are other items that sound cool, but are cursed without a "real" version existing. Those two items give no AC bonus at all. Other than tying up that slot for nothing, there do not appear to be any other penalties.

ah ok, makes sense i suppose. Guess I'll have to pay at the temple to have those removed