WCIF...

[ilovecatsandsims]

This may not even exist; a potion that, when drank by a sim, erases all their memories, relationships, skill points, hobby enthusiasm points, deletes all aspiration points and influence, resets the aspiration benefits, and re-enables first kiss. Like a potion that gives a sim permanent amnesia, so that they forget their identity. (But doesn't delete their family tree)

also...

a potion that does absolutely nothing but is drinkable and has all the special effects of other potions.

Anyone?

[SenkoTwiik]

The first one I am almost positive does not exist. But there is a chance that I am wrong.

The second one... all I can think of is the ingame re-nu-yu potion. It doesn't do 'much' but give your sim a bit of a pick-me-up.

[Theraven]

I think there might be a spell (from AL) that does some of this. I tested the evil spells yesterday, and one of the sims ended up with everything erased from his memory.

EDIT: It's the "tabula rasa" spell - it erases the entire memory of the targeted sim and sets all relationships to 0

Here's a list of all the spells, makable stuff and potions. You might find some usable stuff here : http://www.snootysims.com/infoindex.php?id=magic2

[Tenshii~Akari]

Hmm... I'm not sure either of what you're looking for exists as a mod yet.  To be honest, I don't think there's ever been an in-game custom mod that actually edits/erases memories on purpose... but I'd try what theraven suggested if you have Apartment Life.  

And for future reference, please don't forget to put what you're looking for in your title also, since it's a section rule.  (i.e. WCIF:  Amnesia Spell)  You might just catch someone's attention who may actually know exactly what it is that you're searching for, but usually skips over vague titles.  That, and not doing so would put your thread at risk for either being locked or deleted, since it's pretty much explained in the sticky at the top of the forum.  Just a friendly heads-up.