musn't have done can't have done

can't have done 表示过去的“可能”
例如 He can't have missed the train
他不可能没赶上那列火车
而must have done 也表示对过去的推测
但是否定却是can't have done
也就是说musn't have done 没有这种用法

musn't have done 太严肃了，禁止做考试中不常用。
can't have done表示对过去的猜测，考试中还是比较多的

musn't have done = must not have done (一定不能做)
can't have done = can not have done(没有能力做)