A defective device in my mind would have some obviously fatal flaw - it wouldn't even start up, parts would be broken and dangling off, etc. QC immediately rejects it and it doesn't get incorporated into more complex systems.

A buggy device is much worse for any critical application - it appears to work under inspection, and even limited testing, but 1% of the time it develops some fatal data race condition that causes it to fail erratically and cause havoc, e.g. the Therac-25.

So buggy is a subset of defective but it's even worse?