The NAKACK2 protocol has an invalid format string in one of its trace logging outputs:

log.trace("%s: my_highest_rcvd (%) < stability_highest_rcvd (%): requesting retransmission of %s",
local_addr, my_hr, hr, member + "#" + hr);

This results in an error in the log:

ERROR - Illegal format string "%s: my_highest_rcvd (%) < stability_highest_rcvd (%): requesting retransmission of %s"

The % should be changed to %% to avoid the error. The arguments are in fact printed alongside the message however anything with ERROR in the text makes QA nervous.