Can't find the source right now, but in one study on e.coli, over some 20 or 40 years, IIRC, starting from a single parent, with generations divided and some preserved for later analysis, they ended up with a beneficial mutation that was enabled by a prior neutral mutation: those without the neutral mutation did not evolve the beneficial one, those with did.
You're thinking of
this.
But I don't know if it's been determined yet that the original mutation (~ 20,000 generations) was
neutral, just that it didn't allow metabolization of citrate.
Research continues ...