SAN-SFO on a United 777

I was looking at ACARS of SFO and found this:

ACARS mode: 2 Aircraft reg: N799UA [Boeing B772]
Message label: 5Z Block id: 1 Msg no: M29A
Flight id: UA0954 [SAN-SFO-LHR] [United Airlines]
Message content:-

What gives?

Could be an error. Looking at FlightAware’s history of the flight, the routing is LAX-SFO with a B733 then a change of aircraft to a B772 for the SFO-LHR segment. The SFO-LHR segment, however, isn’t operated all of the time.

What is the date on the ACARS?

March 22,2009

the SAN-SFO segment was canceled that day.

Nope. The flight is LAX-SFO-LHR. It doesn’t originate in SAN.
Airline Flight Departure Arrival

Here’s the straight dope from

United Flight 954
Los Angeles, CA (LAX)
Sun, Mar 22
Scheduled: 9:51 AM
Actual: Flight cancelled
Reason: –
Terminal 7
Gate: 71A
San Francisco, CA (SFO)
Sun, Mar 22
Scheduled: 11:23 AM
Estimated: Flight cancelled
Reason: –
Gate: –
Baggage claim: –

Flight 954
Arrived at Gate
San Francisco, CA (SFO)
Sun, Mar 22
Scheduled: 1:31 PM
Actual: 1:26 PM
Reason: –
international terminal concourse g
Gate: 101
London, United Kingdom (LHR)
Mon, Mar 23
Scheduled: 6:55 AM
Actual: 6:35 AM
Reason: –
terminal 1
Gate: 149
Baggage claim: –