Science Fiction & Fantasy Stack Exchange is a question and answer site for science fiction and fantasy enthusiasts. It only takes a minute to sign up.
Sign up to join this community
Are there any instances in Harry Potter, where it is theoretically known that a spell with a particular effect exists, but nobody knows how to actually cast it?
An example could be that wizards know how to cast a 'heat ray' spell and can deduce by some in-universe rules that a 'cold ray' spell must exist, but nobody can work out how to do it.
Yes
There do exist some spells in Harry Potter that only Voldemort could do such as -
Using the highly advanced art of spell creation, Voldemort was able to create four spells/magical items unique to him alone. These included being able to fly under his own power, creating the infamous drink of despair, the rudimentary body potion, and morsmordre. Theoretically, Voldemort could have done even more which just makes him scarier.
These four spell were known to Voldemort alone and now that he is dead nobody knows how to cast them.There was the spell of Unsupported Flight that Voldemort taught to Snape but he's dead too.
Then there is his rebirth -
Although many have tried, Tom Riddle is the only wizard to achieve complete rebirth. During his lifetime, Tom Riddle split his soul so many times that, when he was finally beaten, there was barely any humanity left. He was just a cursed soul trying to survive, which ultimately led to the scar on Harry's head.
Due to his obsessive fear of passing away, Voldemort was able to put into action a number of contingency plans should he be stripped of his physical self. After interacting with Peter Pettigrew though the ritual, Voldemort was reborn as his physical self, becoming the first and only to do so.
So another ability that is possible but since Voldemort is dead nobody knows how to do it.
No
Spells are invented by a wizward trying to achieve a specific effect. There are a lot of mentions throughout the books about wizards that are especially good in inventing new spells:
Behind this link you can find a much longer list
And there are wizards who are not that good at all: look at Lunas mother who killed herself trying to invent a new spell
...she did like to experiment and one of her spells went rather badly wrong one day. I was nine." — Luna Lovegood, explaining her mother's death.
So if there was no wizard to invent a spell then there would be no spell.
If your question was: Are the EFFECTS of a spell that logically should exist but nobody came up with the correct spell yet: Probably... but we do not have enough information or any mention in the books for them.
But there are good examples for people looking for such spells. Take St.-Mungo hospital: They search for a cure for Nevilles' parents and to reverse the effect of the memory charm that fired back on Gilderoy Lockhart. Although I admit that this does not necessarily have to be a spell, it could also be a magic potion.
