CTLR ISSUED INCORRECT ALT TO A B737 DEPARTING LAX.

Narrative

ON LAX RWY 4 DEP APPROX 5 MINS AFTER TKOF; SOCAL HAD CLRED US TO 17000 FT. AFTER A BRIEF PAUSE ON A HEADING OF 160 DEGS; ISSUED US AN EASTERLY HEADING IN CONJUNCTION WITH 'DIRECT THERMAL WHEN ABLE.' I WAS FLYING WITH LNAV ENGAGED AS WELL AS MCP SPD; I BELIEVE. PASSING JUST OVER 16000 FT; SOCAL INSTRUCTED US TO 'MAINTAIN 15000 FT.' CAPT RESPONDED WE WERE PASSING 16000 FT. SOCAL CAME BACK WITH 'MAINTAIN 15000 FT.' I DISENGAGED THE AUTOPLT; RETARDED THE THROTTLES; AND BEGAN AN IMMEDIATE DSCNT. DURING THIS TRANSITION; SOCAL ISSUED A HEADING OF 050 DEGS; TOWARD WHICH I INITIATED AN IMMEDIATE TURN. I NOTICED THE CAPT HAD RESET THE ALT WINDOW TO 15000 FT; SO I REENGAGED THE AUTOPLT IN HEADING SELECT MODE TO ALLOW IT TO LEVELOFF FOR ME; WHILE I VERIFIED THE MASTER CAUTION WAS INDEED FOR AN OFF-SCHEDULE DSCNT. I DO NOT REMEMBER IF SOCAL EVER ACTUALLY ISSUED A TFC CALL. HIS TONE CHANGED FROM EXCITED TO APOLOGETIC IN SHORT ORDER; INDICATING TO ME THAT THERE WAS NO SERIOUS CONFLICT; AND THAT HE HAD REALIZED HIS MISTAKE. IN FACT HE USED THE WORDS; 'MY MISTAKE;' AND THANKED US SINCERELY FOR OUR HELP WHEN HE SWITCHED US TO ANOTHER FREQ. THERE WERE NO TA'S OR RA'S. I BELIEVE THIS TO BE ONE OF THOSE AMAZINGLY RARE OCCURRENCES OF FLAT-OUT CTLR ERROR. WHEN THE PROB BECAME KNOWN; ALL PARTIES ACTED IN A CALM; AND PROFESSIONAL MANNER TO RESOLVE IT. COM WAS CLR AND CONCISE. CORRECTIVE ACTION WAS PROMPT AND AGGRESSIVE WITHOUT THREATENING FLT ATTENDANT INJURY. PNF DID A SUPER JOB OF SEEING HOW TO HELP WITH THE MCP PANEL; ALLOWING PF TO REENGAGE THE AUTOPLT AND SEARCH FOR ANY CONFLICTS.