Prove that the zeroes of a Harmonic function is never isolated. All I can think of is a very rough idea of a proof by contradiction.
Taken from some old lecture notes (note, I believe this only applies to real valued functions):
Suppose $u$ is harmonic and real valued on $D$ with $a \in D$ and $u(a) = 0$. Take $r \gt 0$ such that $\bar B(a,r) \subset D$. As the mean of $u$ over $\partial B$ equals zero, either $u$ is identically $0$ or $u$ takes both positive and negative values on the boundary. But having both $\pm$ values on $\partial B$ implies $u$ has a zero on $\partial B$ which implies $u$ has a zero on the boundary on every sufficiently small ball centered at $a$. Hence $a$ is not an isolated zero of $u$.