Odd error message about SIP CANCEL request(?)

Running * 1.4.0- I started noticing this WARNING message fairly frequently in my logs:

chan_sip.c:12175 handle_response: Remote host can’t match request CANCEL to call ‘(call id/ip)’. Giving up.

I haven’t been able to find any references (other than in source code) to it either here or via Google. Any ideas? My SIP config isn’t much:

[general]
rtcachefriends=yes

;The following is for QoS:
tos_sip=cs3
tos_audio=ef

;Everything else is handled by the sip table in the Asterisk MySQL database...