For some things, there can be only one: first, best, least, most, and the list goes on. But that does not stop people from using these words incorrectly in a sentence.
Sometimes, it is permissible to exercise poetic license as in the song “It’s A Most Unusual Day” (words and music by Harold Adamson & Jimmy McHugh).
However, I do not think that the erudite and highly respected Charles Osgood should be saying “… a most unusual …” on his Sunday morning broadcast (aired 11/25/20120. There can only be one “most” so to say “a most” rather than “the most” would be incorrect.
“CBS SUNDAY MORNING WITH CHARLES OSGOOD was the #1 Sunday morning news program in households, viewers and adults 25-54, according to Nielsen live plus same day ratings for Oct. 7, 2012. CBS SUNDAY MORNING posted a 3.5/09 with 4.93m viewers and a 1.3/06 in adults 25-54.” Source: CBS press release