Grammar books routinely insist on "the most" as for all superlatives, but I can recall certain cases where 'most' has not been used as 'the superlative' but only as 'a superlative!'
(as in)
Mr. Simpson said, "this is a most irregular way of writing this sentence."
He is a most brilliant exponent of the bamboo flute.
But for her extreme lack of discipline, she is a most likeable woman.
(tangential musing: how important is a comma, or its omission! Pl. compare these two constructions --
"but for her extreme lack of discipline, she is a most likeable woman"
"but, for her extreme lack of discipline, she is a most likeable woman.")
Added after 15 minutes by edit:
I also recall seeing least and other superlatives being used similarly, as in
She racked her mind for a least offensive word to describe his vile deeds to his children.
They looked for an easiest and simplest way to convince the old person.
Smoking was banned at the Simpsons' -- they considered it a worst offence.
Grammatical giants at EL & U! Do you consider such cases odd usage? (or even downright wrong, I shouldn't be surprised!)