António de Oliveira Salazar served as prime minister and dictator of Portugal for 36 years. Following a head injury he was removed from office, but no-one told him and he died two years later still believing himself in charge.

Up until the Carnation Revolution of 1974, Portugal was ruled for decades by a a nationalist corporatist government that bore some similarities to Italy under Mussolini. For most of that time the government was led by Prime Minister António de Oliveira Salazar. He’s still a divisive figure in Portugal, hailed as either its saviour or its fascist dictator. Salazar’s rule began in 1932 and continued right up until 1968, when he fell off a chair (or bathtub; accounts vary) and developed an intracranial haemorrhage.
Most people with this kind of bleeding on the brain die quickly, and Salazar was quickly replaced as leader by another member of the same party, Marcelo Caetano. But Salazar actually recovered… partially. Extremely ill but apparently quite lucid, the doctors were concerned that a sudden shock would be very dangerous to his health. So no-one told him that he had been permanently replaced as leader.
For nearly two years Salazar lived on in the prime minister’s official residence, São Bento Mansion. He was kept appraised of European and world affairs, and continued to receive visits from his “ministers” as if he were still in charge. A reporter managed to gain access to the mansion and interviewed him, hoping to find out just how much Salazar knew about his situation. After a little poking and prodding questions, the reporter asked Salazar’s opinion of Caetano. The former dictator replied
I know Marcello Caetano well. He was my minister several times and I appreciate him. […] as you know, he is not part of the Government. He continues to teach law at the university and writes to me sometimes, telling me what he thinks of my initiatives. He doesn’t always approve of them – and he has the courage to tell me. […] He does not seem to understand that, to act effectively, to have weight on events, it is necessary to be in the Government.
Salazar acreditava que ainda era chefe do Governo (via Google Translate)
Salazar died in 1970; Caetano was overthrown four years later and by 1976 Portugal returned to democratic rule.
[Thanks to a few people on Twitter for drawing my attention to this topic, including this prophetic Tweet from 2018.]