One example is printing to printer on the serial port. If you want to instantly stop printing you may try turning off the printer. The attempt to resume would happen even if you used a command to stop the printing. The problem was that once the printer software sent bytes to the large serial buffer to be printed, these bytes were not removed from this buffer when the print job was canceled. This will drop all modem control signals from the printer and empty the buffer.
Modern printers have large buffers and often a button on the printer to empty the buffer.
That is bad news if you try to use ttyS3 at this IO address. Another story is that Linux will not detect your internal modem on ttyS3 but that you can use setserial to put ttyS3 at this address and the modem will work fine. But a status register of the UART doesn't get updated fast enough to reflect this. Thus no byte is sent to the port to be transmitted and the UART transmitter waits in vain for a byte that never arrives. There is a proposal to fix this by patching the serial driver.