A freezer or refrigerator excursion is really a never event; by investing a quality storage unit, and taking appropriate steps to prevent unplugging, there should be no reason why it happens.
In spite of this, it has happened to me four times in my career. Why is that? For me, it was because the companies owning the pharmacy I was managing did not invest in a medical refrigerator, so I was stuck handling excursions until I threw away enough medicine they realized it costs less to buy a medical refrigerator.
Investing in a medical refrigerator, like this one from am-1, can prevent thousands in losses due to damaged medications and vaccines.
I never intended to become an expert in vaccine storage and handling but because of these experiences (and my subsequent anxiety in the pharmacy about it happening again) I have spent more time with the CDC Vaccine Storage and Handling Toolkit than I care to admit.