Maybe the employees didn’t clean the ice cream machine properly so the stuper-visor thought this was a good consequence to “teach them a lesson.” DQ needs to brace for a big lawsuit.
I think you’re right, that someone didn’t clean the machine properly and left some sterilizer in it. Back when I worked in fast food, 50-some years ago, the manager would have found out who did it and made sure that person never did it again, possibly by administering physical therapy behind the carwash next door. This DQ manager really screwed up, and will pay for it.