Engineering: design idiosyncrasy: lid can fall into reservoir

Self-tutoring about engineering of consumer products: the tutor mentions a discovery.

I was using a water heating device yesterday whose reservoir has a lid. I dropped the lid by accident, and it fell into the reservoir. I was surprised such was possible.

Normally, in my experience, the cap of a reservoir is designed bigger than the opening it protects, so it can’t fall in. Yet, not in this case.