ALT ALERT SYS NOT SET TO CLRED ALT AND ALT OVERSHOOT OCCURS.

Narrative

AFTER LEVELING AT 14000 FT; APCH ISSUED A DSCNT TO 12000 FT. COPLT WAS FLYING ON AUTOPLT; AND I THOUGHT HE SET 12000 FT IN MCP; AND BEGAN A VERT SPD DSCNT. I READ BACK OUT OF 14000 FT FOR 12000 FT. HE APPARENTLY HEARD 11000 FT AND SET THAT. I DIDN'T CATCH THE ERROR; THOUGH I THOUGHT WE BOTH POINTED AT THE ALT WINDOW. HE THEN CALLED FOR THE APCH CHK; WHICH I ACCOMPLISHED. WHEN IT WAS COMPLETE; I NOTICED THE ACFT LEVELING AT 11000 FT. I SAID 'WE'RE ONLY CLRED TO 12000 FT;' AND ABOUT THAT TIME APCH CTL ASKED US OUR ALT. I RESPONDED WITH 11000 FT AND HE SAID TO MAINTAIN IT. TCASII SHOWED TFC AT 11 O'CLOCK AT 5 MI CLBING; DEPARTING ATL. THE CLRNC OUT OF ATLANTA IS TO 10000 FT; AND WE NEVER LOST SEPARATION. I SUSPECT FATIGUE WAS A FACTOR IN THIS OCCURRENCE. WE HAD ALREADY FLOWN FROM DFW TO LGA; SWITCHED PLANES; AND EXPERIENCED A MECHANICAL PROB IN LGA AFTER PUSHBACK. THIS INVOLVED A LENGTHY PROCESS OF PROCS; RADIOS; MEL; PAX; ETC; WHILE TAXIING AROUND A VERY BUSY ARPT; AND RESULTED IN 1 HR AWAY FROM THE GATE. THIS WAS FOLLOWED BY ANOTHER ACFT SWAP; AND MANY PAX CONCERNS. WE DEPARTED LGA APPROX 2 HRS LATE; AND WERE PRETTY TIRED ON ARR IN ATL.