Is it really bad to take expired b12 shots? what can be a consequence for doing this?

It can be OK: Medications aren't guaranteed to keep their potency beyond the expiration date. It's not dangerous, but simply may not be as effective.

Not bad: Most expiration dates are to satisfy legal and other criteria. The medication may gradually lose potency over many years but you are safe to take something expired. This has been tested by the us military so they do not have to replace their two year supply of medication just based upon expirations which saves them money.