This has actually happened a few times. One example was the 29th Infantry Division. Its infantry battalions designated for the Normandy beach assault spent so much time training for fighting across the beach regular tactics were neglected. The old hands were very stale, and the more recent conscripts and many of the very junior NCO had negligible training for the fighting 500 meters inland and the rest of the way to Germany. Like most undertrained soldiers in history they had to learn the hard way.
When I first joined the USMC in 1974 the corps was going through a remedial training program for conventional wars. Since 1966 there had been a increasing emphasis on the specifics of fighting in Viet Nam, until many Marines had no clue about other battlefields or tactics. Three plus decades later when the US Marines withdrew from the Iraqi mission it was found the artillery battalions had spent so much time on the counter insurgency effort the junior artillery officers were not qualified for the complexities of battery, battalion of brigade artillery missions. Again remedial training programs had to be instituted.
When rifles were first introduced in the 18th Century Assorted armies allowed the commanders of 'Rifle' legions, regiments, companies to train them exclusively as snipers. That filed several times when conventional line infantry or cavalry were able to run down a body of rifle men. lacking sufficient training in bayonets, anti cavalry tactics, ect... the rifle units were often massacred. Successful rifle units were given a more balanced training, and used best when line infantry could support them.
