[htabdoolb]

..You finally destroy a gargoyle in a difficult, close fought battle, and then its only drop is an Eye of Serilis.

Or..

..The Goblin Archeologist you've just slain drops a regular brass mask, in addition to his crappy pack, just to get your hopes up before dashing them.

Or..

..It turns out that the Decaying Skeleton you've just made re-dead was only wielding a Worn Great Staff. Again.

Or..

(You get the picture.)