Cause

This message means that the Zilog 8530 character input silo (or serial port FIFO) overflowed before it could be serviced. The zs(7D) driver, which talks to a Zilog Z8530 chip, is reporting that the FIFO (holding about two characters) has been overrun. The number after zs(7D) shows which serial port experienced an overflow:

zs0 - tty serial port 0 (/dev/ttya)
zs1 - tty serial port 1 (/dev/ttyb)
zs2 - keyboard port (/dev/kbd)
zs3 - mouse port (/dev/mouse)