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The ingredients I am referring to are lacewing flies and boomslang skin. The two ingredients that are usually mentioned in the brewing of the Polyjuice Potion.

However, in the Cursed Child, Professor McGonagall mentions that these two ingredients were stolen from the school's stores, but since they are not on the "restricted list" it wasn't a big deal.

Harry and Hermione do not make any connection even though they have brewed the potion before (Chamber of Secrets), been fooled by it (Goblet of Fire), and used it as a battle tatic (Deathly Hallows). Snape even accused Harry of stealing the same ingredients and saying he was going to figure out why Harry was brewing a Polyjuice Potion.

I think there are two options here...

  1. The "name drop" was a clue to the reader that Polyjuice was afoot and not all was as it seemed.
  2. lacewing flies and boomslang skin are common ingredients.

I am looking for proof of the second option. Are there any other potions known to contain these ingredients? If not, why did none of our heroes (who again are very familiar with the potion) not make the connection?

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Definitively, no, Boomslang skin and lacewing flies are not ingredients in any potion other than Polyjuice Potion. Therefore, they are not common potions ingredients.

I read through the ingredients of every Harry Potter potion, as listed at the Harry Potter Lexicon. As stated, only Polyjuice lists Boomslang skin and lacewing flies as necessary ingredients. However, many of the potions did not have any ingredients listed, but rather listed the descriptions and effects of the potions. I should probably note that the Lexicon's list of Harry Potter potions included those from the books, from Pottermore, and from the Book of Potions from the Harry Potter video game (I think it's the fourth video game -- J.K. Rowling wrote the Book of Potions especially for this game.). The Lexicon doesn't archive any information about the movies, just as an FYI.

So that was my long-winded way of explaining that although no potions aside from Polyjuice require Boomslang skin and lacewing flies that we know of, the lack of full descriptions of all the Harry Potter potions leaves open the possibility that other potions use those two ingredients as well, but J.K. Rowling just hasn't revealed which ones.

On a side note, personally, I think fluxweed may actually be the most "difficult" ingredient to procure for Polyjuice, for the description specifies the fluxweed must be harvested at the full moon, and a moon cycle is slightly longer than the twenty-one days required to stew the lacewing flies.

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    Book of Potions was a video game made for the Sony Wonderbook in 2013 as part of the Sony-Pottermore partnership. It was a sequel to Book of Spells. If you're interested in the contents, I've compiled all of the Rowling bits into a nice pdf. – ibid Aug 7 '16 at 18:40
  • @ibid -- Oh, thanks for the link! I've bookmarked it for future reference and will download the pdf as well (I'm assuming that would be okay). Much appreciated. :) – Slytherincess Aug 7 '16 at 19:32
  • If you haven't yet done that with the one for Book of Spells, you should do so. (Not that it needs to go first, just that the material in it is much better.) – ibid Aug 7 '16 at 20:13
  • @Slytherincess - I don't dispute your answer (competing with mine as it is lol) but can we say they are "definitively rare" when I have a quote explaining that lacewing flies along with a string of other instruments are easily found in the school's student store cupboard? – ThruGog Aug 8 '16 at 13:40
  • @ibid - Nice pdf! I played Book of Spells but never Book of Potions, and you've now sorted that gap in my Potter knowledge. Thanks. – ThruGog Aug 8 '16 at 15:59
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I've done a few searches on this subject and it would seem they are only mentioned for Polyjuice.

However:

The lacewing flies must be stewed for 21 days to go into Polyjuice, perhaps suggesting that they can be used in different ways if not stewed for such a specific amount of time. In CoS (page 124 UK version), Hermione says, "They're easy enough, they're in the student store-cupboard, we can help ourselves," about these, leeches, fluxweed and knotgrass.

The boomslang skin and the bicorn horn were the only items stolen from Snape's stores, definitely indicating that they are the more rare of the ingredients. Before this decision is made, Hermione says on the same page, "That'll be tricky," and Harry complains, "That's definitely not in the students' cupboard. What're we going to do, break into Snape's private stores?"

So, in conclusion, it is up for debate whether the characters should have recognised the combination or whether lacewing flies are so common that even this pairing doesn't mean much.

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