@mdpye I'm glad your flatmate or anyone else wasn't injured. And for me this is the big one, no one gets hurt. So I would go to the shop when calm and explain what happened, don't blame don't accuse, don't go looking for retribution or compensation.
Make them aware of what you believe has happened and ask them if could be aware of this with future customers. No one is perfect, but mistakes do happen. A no fault approach to improve future service and safety for others.