Just outside the north edge of Great Smoky Mountains National Park is a scenic Tennessee highway connecting I-40 near Newport to Gatlinburg and Maryville. During the late 1970s and early 1980s, it appears TNDoT was determined to unify this corridor with a single US route designation. They first attempted to accomplish this by extending the US 74 designation west from Asheville NC (which was the route's western terminus from 1926-1987). Following is the route description from the actual minutes of the June 1978 meeting of AASHTO's route numbering committee:
Notice about where the proposed route re-connected with I-40 west of Knoxville, it would have made a strange curve back around to the northeast. I suspect this was because Tennessee also wanted to have a US highway serving the city of Oak Ridge. At any rate, AASHTO disapproved this petition at that meeting. Possibly this was because NCDoT was not a co-sponsor on the proposal, and they had not yet extended the US 74 designation to the Tennessee line.
Tennessee did not give up, although apparently they did concede that the idea of running their proposed US route back through Oak Ridge was a little weird. Their next request was for a stand-alone US route (unspecified number) represented by the thick red line on the map above. Note that this second proposal was coincident with the original proposal from approx. Cosby to Lenoir City. AASHTO rejected this proposal at their June 1981 meeting. I suspect the main reason was because it would have been a new US route entirely in one state.
Tennessee quickly addressed that issue by putting together yet another proposal. This one extended the US 321 designation southwest from Johnson City to Newport, via an approx. 60-mile overlap with US 411. From there US 321 was routed down to the originally-proposed corridor and westerly to I-40 at Lenoir City. AASHTO approved this proposal at their October 1981 meeting...
...although they should have rejected that one too. Extending US 321 like that was awkward. It caused some logistical complications, as well as collateral damage to previously-existing US routes in the area. Moreover, it was unnecessary: if Tennessee wanted to unify their scenic corridor, they could have done so by simply assigning it a state route number. More details can be found on the main US 321 page.