While in decorate mode, sometimes an item will suddenly disappear from where you're working on it and you'll find it elsewhere in your house. It seems to be particular to containers.
What is happening is that I will be dropping one bag and picking up another. Instead of drag-clicking I'll accidentally one-click on a bag and it'll teleport to the last place that I have dropped a bag instead of staying where it is. It is very disconcerting when it happens to be off the screen.
Another variant is when I'll be dropping a bag on the main floor and get it a little below the floor. Instead of "settling up" to floor level, the bag will whisk away to the lower left corner of the house interior.
I can't get it to happen consistently, but I can get it to happen once every few minutes of dropping and then rapidly clicking on another bag.
I have also had bags end up way out in the yard in their teleporting. I thought I'd lost 3 bags testing this, but found them well to the left of my house.