Perhaps not mission critical small, it might be in the range of 0.1-2% risk for each landing/takeoff cycle. So if you require 10 refuelling launches, your total risk is 1-20%.

Say you had a large moon lander, or multiple landers that make up a robotic fuel collection and refining system. Then you have a reusable lander-launcher that lands, collects the fuel and delivers whatever it doesn't use for takeoff/landing into a orbital fuel deport orbiting either the moon or earth. Actually might be best to have your deport orbiting the moon until it's full then send it on a slow, low-fuel, multi-month path back to earth orbit, perhaps you meet the Mars transfer vehicle there.

If at any point the fuel transfer lander-launcher fails, you can launch a new one from earth and push back your Mars mission. Perhaps you have multiple fuel transfer landers as a contingency, or to increase throughput (fuel in orbit isn't just useful for a single Mars mission)

Only once you have your fuel ready, do you launch your Mars crew.

You could do a similar setup at Mars to collect fuel for your trip and avoid having to drag that fuel from the moon. But a Mars fuel transfer lander-launch would be much more complex and risky, and it would be a lot harder to land the fuel collection/refining facility.