To crack a mirror is to provide an opening for the reflection to escape.

Shards of Alara

Minion Reflector

Whenever a nontoken creature comes into play under your control, you may pay {2}. If you do, put a token into play that's a copy of that creature. That token has haste and "At end of turn, sacrifice this permanent."

Whenever a nontoken creature you control enters, you may pay {2}. If you do, create a token that's a copy of that creature, except it has haste and "At the beginning of the end step, sacrifice this permanent."