Well, there was that famous time in Rio de Janiero when the conductor quit, a substitute was booed off the podium, and one of the cellists (who was late because he’d been “rehearsing Schubert lieder” with one of the contraltos in the chorus in his pensione) took over the performance. The cellist, of course, was Arturo Toscanini.
I think a lot of orchestral musicians can be at least capable conductors in an emergency, and of course they should know the piece pretty well. I’ve heard quite a few concerts conducted by violinists. The hard part, I think, is replacing soloists.