Can GRH for complex primitive Dirichlet character fail with a single non-trivial zero off the critical line?
For real characters this is impossible because the non-trivial zeros are in quadruples.
On the other hand this paper in Russian constructs a function very similar to zeta with exactly one quadruple of zeros off the critical line.
The motivation: GRH implies a certian efficiently computable sum over zeros of $L(s,\chi)$ must be $0$ and experimentally it is (indistinguishable from) $0$ and I wonder if this result is trivial even if GRH is false. IMO the sum should not be zero if the answer to the question is 'yes' or some cancellation doesn't happen.