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(unlikely event of) emergency; the worst(-case scenario); 10000 to 1
(if) by some chance; by some possibility; in the unlikely event that
万一に備えて保険に加入しておいた。
I took out insurance as a precaution against emergencies.
万一の場合に連絡してください。
Please contact us in case of emergency.
万一のことを考えると心配で眠れなくなる。
When I think about the worst case, I can't sleep from worry.