Linguistic Note on Invalid
The word invalid, as used in the logical context, has the stress on the second syllable: in-val' -id.
Pronounced in' -val-id, with the stress on the first syllable, the word means a person who is being treated for an illness.
There are plenty of apocryphal tales on the internet where furious users have called the helpdesk to tell the staff that their computer is calling them bad, and an invalid.
So let's be careful out there.