Once in Charlotte (back in US days) our cabin crew was delayed so they brought in a completely different crew to board the flight, then our "real" flight attendants took over once they arrived.
I've heard gate agents say "we can't leave because the crew hasn't shown up" or "we don't have pilots" (throwing the cockpit crew under the Airbus, you might say) and have seen social media gripes based on this ("Hey AA, tell your pilots to show up to work on time!" - that sort of thing) - I'm willing to bet that the vast majority of the time the real situation is that the pilots are impacted by a previously delayed flight.