I couldn't think of a decent title for this weird problem.
When I started this fort, I brought an insane amount of wood, and built fourty beds for my barracks straight off. I set them to be built once my barracks was dug out, but since my dwarves were mostly busy doing their own thing they only planted a few of the beds.
Later on, when immigrants came, I had already dug out rooms for my nobles. I decided to check if I had any free beds, and it turned out I did; it said I have five beds. I decided I must have accidentally built more beds than I thought, and used three of the beds for some of my noble quarters.
The immigrants started building the many beds, they didn't have much else to do. The problem was, I started getting errors that said "Item nolonger exists." I quickly realized I really HAD only made fourty beds, and the three I had used on the noble quarters has somehow been 're-offered' for my building, even though they were already queued up for another building.
I also built a number of doors early on in my fort, but the dwarves never bothered putting them up. Only a couple of my original seven had furniture building on, and one of them was busy guarding my outdoor stash from kobolds. I'll see if the same problem crops up with those doors, now.
Edit: It just happened for one of the fourty tables in my barracks attached dining room; I had placed an 'extra table' in another room for my merchant to use as an office, but it wasn't extra after all.