Some enemies have the power to disarm you and make you drop your weapon; my current character is a mage who uses a Bow and on occasion I get hit with a power which disarms me.

Usually a quick search and you can find it again but I have needed to reload a previous save sometimes when I can't find it.

I gave my follower the Dragonbane sword as its not a bad weapon on its own plus its good against dragons so if we ever saw one then they would have a good item to help take down the dragons health.

I checked my followers inventory recently and the Dragonbane sword is no longer there so I can only presume that they must have dropped it during one of my recent missions. I would have thought that if a follower dropped a weapon that they would also look for it and pick it up again just like I would but obviously on this occasion they didn't which just means I have lost a good unique weapon.

What are some good items that you have lost either this way or some other way in which you are annoyed that you lost it?

The Ebony Blade
I gave it to a Thrall and he suddenly vanished later. I'm still upset as that was the sword I was going to use after I leveled up one handed.