Once would be one thing, but being late twice is unforgivable.
子供ならまだしも、大人がそんなことをするなんて信じられない。
It might be understandable for a child, but I can't believe an adult would do such a thing.
If it happened once, that would be one thing, but making the same mistake repeatedly is a problem.
Being late is one thing, but an unexcused absence is unacceptable.