This is nasty - if you are building and are editing an object, then go to your personal inventory and select something in it, then hit the delete key, the object you have open for edit is deleted, not the inventory item.
I believe this is just a problem with the focus not correctly being switched, and I also think this was fixed a couple of releases ago and this behavior is a regression. I bug reported this.
BTW, if this happens to you, the deleted object goes to your Trash folder.