In 1882 Kronecker proved that every algebraic subset in $\mathbb P^n$ can be cut out by $n+1$ polynomial equations.
In 1891 Vahlen asserted that the result was best possible by exhibiting a curve in $\mathbb P^3$ which he claimed was not the zero locus of 3 equations. It is only 50 years later, in 1941, that Perron gave 3 equations defining Vahlen's curve, thus refuting Vahlen's claim which had been accepted for half a century.
Finally, in 1973 Eisenbud and Evans proved that $n$ equations always suffice to describe (set-theoretically) any algebraic subset of $\mathbb P^n$