Unfamiliar console message: in prompt_tty(): caught signal 2

On the serial console on a Pi3 v1.1 (so armv7) I just noticed an
unfamilar message:

Apr 20 22:14:37 www su[30398]: in prompt_tty(): caught signal 2

Several login failures were reported shortly afterward, so
the message seems to have been a console message, not from
the tip session used to connect.

I've never seen it before and wondered if it has any special 
importance. The machine was running buildworld on -current,
updated a day or so ago.

By the next morning the machine had locked up hard, no
response to the enter-tilda-control-B debugger escape.

After power-cycling it came back up after fsck  and buildworld
was resumed where it left off.

Thanks for reading, 

bob prohaska