It the military we had a way of characterizing risk in a severity and probability matrix. I think it helps to think about things like that.
MCAT 25 or less (first time taking) - Severity: Marginal (you can retake and recover), Probability: Occasional
= Medium risk
Step 1 failure (first time taking) - Severity: Catastrophic (your residency chances are now limited and you must pass on retake for a decent chance at any. Also you will possibly have to delay starting 3rd year and maybe take a year off). Probability: Remote
= Serious risk