Tutoring English, familiar words can hold surprises. The tutor mentions such a case with the words sanatorium and sanitarium.
I almost never hear sanatorium, but rather, sanitarium. They have the same meaning: a facility promoting patients’ recovery, possibly from long-term ailments.
Mish, Frederick C (editor). Merriam-Webster Dictionary. Springfield: Merriam-Webster, 2004.
Jack of Oracle Tutoring by Jack and Diane, Campbell River, BC.