I have been reading James Mellon's Bullwhip Days and one of the interviews states that upon the end of the Civil War a man from the US army came to the plantation to inform the slaves of their freedom. It made me wonder that in cases where nobody came to inform the slaves of their freedom if the slaveholders would simply not inform the slaves and carry on, or perhaps even just force them to continue working anyway.
Yes, sadly, it did happen. To quote an answer I gave here several months ago:
I actually just finished reading Levine's The Fall of the House of Dixie less than an hour ago. He discusses three major responses from disaffected slaveowners.
Firstly, the majority of planters attempted to keep some form of more-or-less legal control over the freedmen, which eventually developed into the Ku Klux Klan, sharecropping, and segregation.
Secondly, some attempted to leave the country. Many went to Brazil, but they weren't able to take slaves with them - Brazil was a long sea voyage away, and after four years of war and blockade, planters wouldn't be able to charter and provision a fleet for themselves (to say nothing of how the international slave trade had been banned for decades.) Also, a party of several hundred under General Kirby Smith and Commodore Matthew Maury went to Mexico, to join the French puppet Emperor Maxamilian. They brought slaves, hoping to start new plantations. However, slavery had long been illegal in Mexico, and not even the Confederate-sympathizing Maxamilian had restored it, so (to quote ex-general Hindman) "all our negroes decided to leave us upon our arrival here." Maxamilian's defeat and execution in 1867 put a final end to this project.
Thirdly, there were some who tried to hide news of emancipation from their slaves. Levine mentions incidents in Texas and Georgia, including one master who explained it by saying Lincoln's death had restored slavery. One African Methodist Episcopal missionary in Georgia said, even four months after Appomattox, "The people do not really know that they are free." This did eventually fail, one plantation at a time, without much fanfare. Levine quotes one newly-freed girl saying "We stayed [on the plantation] another year after freedom" until "we finally found out we were free and left." Sadly, though Levine doesn't speculate, it's unlikely she was the last to learn.
Definitely happened. Witness the claim of Jacob Furguson to the Southern Claims Commission after the war. He states: "I was a slave at the beginning of the war and was a slave some three or four months after the war ended." This was well after returning Confederate soldiers had told of the end of the war. He eventually received compensation from the Commission for work done for Union soldiers during the war.