Has there ever been a fatal case of flatulence?

Yes, there have been cases of fatal flatulence or 'fatal gas expulsion'. An unfortunate and most likely rare case was reported in the British Medical Journal (BMJ) in 2011. According to the report, a man aged 69 died after suffering an explosive bout of flatulence. The incident took place while he was receiving palliative care at home as part of his terminal cancer treatment. The cause of death was determined as perforation of the colon due to a large-volume rectal gas emission which resulted in pneumoperitoneum (accumulation of air within the peritoneal cavity).

The BMJ article further suggests that this is not an isolated case and concludes with saying that "flatulent explosions may result in abdominal trauma leading to fatal consequences." Although such cases are very rare and hard to find reliable evidence for; it appears that even passing gas can be potentially lethal if enough pressure is built up inside one's intestine!