/slaps head/

Quote
“I need something that will give me the appearance of death, something that has been tested successfully with no long-lasting effects, right?”

They looked at each other with growing delight and said together, “Resurrection!”
Of course! It's obvious now that you've mentioned it! It makes total sense! Why didn't anyone else (i.e., the writers of the show, darn it!) remember Resurrection?

From TV Tropes:

Forgotten Phlebotinum:
A situation, most common in sci-fi and fantasy, where an amazingly useful power or device is revealed in one episode, and would be amazingly useful in later episodes, if it weren't for the fact that nobody seems to remember it. Sometimes the power or device is remembered under circumstances where it proves mostly useless, but not remembered when it would do any good.

This is for powers or devices that are forgotten in general. Something which the character does use a lot and only is forgotten this one time is Plot Induced Stupidity.

If the device is remembered, but there's some contrived excuse as to why it isn't available or won't work, that's Holding Back The Phlebotinum or It Only Works Once.

Not to be confused with We Have Forgotten The Phlebotinum . If they (finally!) remember to use it in the end, it's a Forgotten Superweapon .

This trope does not necessarily denote bad writing. It can be (as noted in the Order Of The Stick example) convenient writing instead. If the protagonists have some piece of phlebotinum that makes them invincible or at least very hard to so much as injure that is both reliable and accessible, vast numbers of plots have to be thrown out the window. Some would call this unwillingness to change the Status Quo and then adapt to the new order of things "lazy", but when one is working on a regular series, changing the status quo (interesting though it can be dramatically) is not something to be done lightly. When it's a Shared Universe this is even more pronounced. In such cases, "Forgotten" Phlebotinum is a subtrope of Real Life Writes The Plot.

When it's not forgotten and is used in a later episode because a writer wants to acknowledge continuity, it is Olaf\'s Hammer.