After hearing yet again what a great prank replacing shampoo with Nair is, I felt inclined to ask if it would actually work. A casual search reveals this prank cited often as a good revenge tactic, but no evidence that it has successfully been performed or would work as advertised.
I haven’t used Nair before, but it seems to me it would require more contact with the hair than what you would normally get from using shampoo.
So does this rank as urban legend? Or has it actually been done?
Don’t know if it’s been done, but Nair usually takes (IIRC) about 10-15 minutes to work: I don’t know of anyone who leaves shampoo on that long. The smell would likely give it away as a ‘foreign substance’, not to mention the difference in texture (feels more like conditioner than shampoo). I could see someone exchanging the conditioner for Nair, though.
As screech-owl says, it would probably be noticed right away because Nair doesn’t look, feel, or (especially) smell like shampoo. Plus, it’s intended to work on relatively fine body hair, not the thick hair on top of your head, and I doubt it would be very effective.
Just want to add that by “it doesn’t look like shampoo”, I mean it may not be the same colour as the shampoo you’re replacing. I never use pink shampoo, but I’m sure it exists so I wanted to clarify my point.
Anyway, if someone actually tried the Nair switch, the reaction would be more like “Hey, who’s been messing with my shampoo?” and no real damage would be done.
The point is to not replace the shampoo completely. You merely add some depilatory into the victim’s shampoo.
Over the course of a few weeks the victim’s hair will become more fragile and break out in clumps. If your target happens to be on his first voyage and he works near the reactor it can break the monotony of almost any mission…
And all of this assumes you get the Nair that doesn’t smell. That would be the biggest tip off: Nair (or at least the Nair I’ve smelled) doesn’t smell like shampoo or conditioner. Or, if you get some Nair that does, you have to make sure it smells like what your victim uses, else the observant ones will know right away.
But like DrFidelius said, if you add just a small amount you may have better luck over a long(er) period of time.
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Any hair remover I’ve ever used/seen smells strongly enough to knock over a horse.
Anyway, that’s passive-aggressive.
Passive-aggressive: the best of both worlds.
But if you want to be confrontational, just tell the victim beforehand that you cursed him. “What kind of curse?” “Oh, you’ll see…”
I like the way your mind works Mr2001! 
The new Nair formula only takes 3-5 minutes to work.
It reeks, but there are also some new(er) formulas that don’t stink quite as much.
But I don’t think it would work as a revenge tactic…unless the person you are getting revenge on is completely ditzy and pays absolutely no attention to what he/she puts in his/her hair.
This is mean! And what if the victim gets Nair in his/her eyes?? 
The the said person is out some eyebrows and eyelashes. And maybe a couple o’ eyeballs.