Least unexpected may in some contexts be the phrase called for. These, for instance, from the first page of Google results on “least unexpected”:
The Least! Unexpected! News! Story . . . !Evah source
The least unexpected award of the night – Meryl Streep gets her gong for The Iron Lady. source
Nobody was surprised that Tom Cruise was divorced and Meryl Streep won the Golden Globe; everybody expected these events, so neither story was at all unexpected. It is only a modest hyperbole to describe these as least unexpected.
On the other hand, this review (also from Google first-page results) clearly has it wrong:
“A Gem in the least unexpected place” ... We were on our way to Grand Tetons and had a long day of driving. We were both thinking lets just find a local "greasy spoon" and check it out ... source
In conversation, or in quick, casual writing, people use the words that occur to them on the spot. Unlikely or unexpected, get jumbled up with least likely, least expected, and the result is least unexpected. Nobody rewrites, nobody's keeping score, both speakers/writers and hearers/readers understand what is meant, so there's no harm done. Such misuses are not in the least unexpected.