This wasn't a construction mistake! This sign on US 160 in Lockwood had the Missouri outline instead of the US shield. It was relatively new in 2002, because it's using one of the wider sign blanks that Missouri didn't use until 2000.
This wasn't the only such mistake on US 160: there's another in Forsyth at Missouri 76.
David Backlin reports this marker was replaced in September 2002 with a standard US shield.
Photo courtesy of David Backlin (taken April 2002)