Manindra Agrawal and Thomas Thierauf used the splitting lemma to give a polynomial-time algorithm for factoring. They got quite excited and couldn't find a bug in their proof. Apparently I suggested to Thomas at the time that he patent the algorithm before they publish. But later Klaus Wagner showed the splitting lemma actually implied P=NP and Agrawal went back and found that the proof of the splitting lemma wasn't correct and Jochen Messner found a counterexample.
Manindra wrote a retraction for Theoretical Computer Science but for some reason it wasn't published.
I've seen this before. If a technical lemma doesn't imply anything particularly surprising, it might not get checked very carefully. But only later when researchers start using it to get amazing results do people go back and realize there was a problem with the lemma after all.
Flash forward more than a decade later to this week at Dagstuhl. Harry Buhrman, John Hitchcock and Harry's student Bruno Loff came to Dagstuhl all excited. They proved that SAT disjunctively-tt reducible to a sparse set implies SAT is reducible to a weighted threshold and by Agrawal-Arvind thus implies P = NP, answering a long-standing open question. We tried to understand the Agrawal-Arvind paper and Thomas Thierauf "vaguely remembered" there might have been a bug in the splitting lemma.
Arvind who is here at Dagstuhl didn't remember the paper well. Manindra, one of the organizers of this week's workshop, is not here at all but had to attend an IIT event in of all places Chicago. Eventually Harry called Manindra who acknowledged the bug.
Once revealed Jacobo Torán, Messner's advisor, told me the whole sordid story above and Nikolai Vereshchagin showed the splitting lemma true for n ≤ 4 and found counterexamples for n ≥ 5. After talking about the conjecture at the open problem session last night, at breakfast Amir Shpilka showed me an easy proof that the splitting lemma fails even if you allow an infinite number of points for n = 5.
Don't worry, Manindra's proof with Kayal and Saxena that Primes are in P is still safe.