- Open "emacs -Q"

  - Eval (make-network-process :name "foo" :server t :service 8080 :family 'ipv4)

  - Press "C-x 5". After a few seconds, the echo area will display "C-x 5-".

  - In another terminal, run "yes | nc localhost 8080"

  - The "5-" in the echo area will begin to flicker.

I've tested this in emacs 25.3 and 26.1.

As far as I've been able to tell, the data coming in to the network process triggers [read_key_sequence] in [keyboard.c] to replay the "C-x 5" sequence. As part of this replay, it outputs "C-x", then "C-x 5", then "C-x 5-" to the echo area, which causes the "5-" to look like it is flickering.