Twice. Once in CO in 1994, me and a buddy watched one for about 30 minutes in the krummholz at about 12,000 feet in the Sangres. State wildlife folks caught wind about it a couple years after the fact and interviewed us; buddy is a well-known biologist and his testimony stood up. The State folks told us there were no known breeding pairs, but that the occasional wanderer would come through...
And more recently, on a January pheasant trip in ND close to the Missouri River - saw one walking through the snow and driving wind in a farm field like it was any old Tuesday.
Neither were marmots, badgers, bears or fishers. Very cool & exciting to see.