I can't be the only one who, since before I installed any mods (and on my old computer, as well), has had sims randomly drop items from their queues (with free will turned off).

It usually happens to me in this kind of situation:
1) Large household (five or more sims).
2) Every sim is told to use their private bathroom (go pee), then sleep in their individual bed.
3) The selected sim does it in that order, as do a few others, but one or two either go straight to bed or drop all items from the queue.

Why does this happen? Why does it never happen to the selected sim?

One thing comes to mind, the ones who get drop queue had the way blocked by another sim and this will make them drop queue items.

if not blocked by another sim, they might be blocked by objects and/or walls/fences. the moveobjects cheat might help with each of those.

or they might be blocked by "invisible" sims.
-possibly another household lived on the lot before the current household. Pescado's Lot Debugger (also known as the bat box) can help in that case; the "clear offworld loiters" interaction.
-possibly some current household member did the meditate kind of teleport at some time. These steps might help in that case::
1. Save and exit the lot.
2. re-enter the lot.

What?! What on earth is an "invisible" sim? Also, mind sending a link?

Explained in the next lines of the post.
in different words:: a sim that is not really there.
2 possibilities for that (like I told in the post)::
-some household living there before the current household
-some current household sim doing meditate kind of teleport

here is the lot debugger I mentioned:: http://www.moreawesomethanyou.com/s....php?topic=72.0

Oh. Thanks, but there were no sims meditating and it was a brand new house... any other ideas? This has been happening to me on multiple save files on two different computers, but it's almost always with large households. It seems like maybe the game can't remember all the tasks, so it drops some, but that makes no sense...

might try any of these (order does not matter)::
- re-arrange furniture.
-add stairs/doors at various places of the house.
-add/remove walls; before removing walls, move wanted objects to other walls. and removing wall cover before removing wall would help create extra money.
-add floors.

Is the house two stories? Does it have modular or fixed stairs? Make sure there is good flow through your house and only use modular stairs. The most likely cause is one of the first sims to move got in the way of one of the last sims. If your house has a lift/elevator that can also be a cause of queue drop, there is a mod at MATY to fix that. I forget what that is called though.