ALTDEV DURING DSCNT TO DTW; MI.

Narrative

ON THE SPICA ARR INTO DTW; ZOB CLRED US TO CROSS SPICA AT 12000 FT MSL. WE HAD 12000 FT IN THE FMS AS A XING RESTR WE EXPECTED. AS WE APCHED SPICA IN THE DSCNT; I THOUGHT THE ACFT OPERATING IN THE MANAGED DSCNT MODE MAY NOT MAKE THE RESTR; SO I SWITCHED TO THE VERT SPD MODE AND INCREASED THE RATE OF DSCNT TO ASSURE THE RESTR WAS COMPLIED WITH. AS THE ACFT LEVELED AT 11000 FT; I LOOKED AT THE FMS ALT OF 12000 FT AT SPICA AND AT THE ALT SELECTED ON THE AUTOPLT (11000 FT) AND REALIZED THERE WAS A PROB. WHAT ALT WERE WE CLRED TO? I ASKED THE CAPT (PNF) WHO HAD BEEN BUSY ON THE RADIOS AND ACCOMPLISHING CHKLISTS. HE HAD DOUBTS AND ASKED THE CTLR IF WE WERE CLRED TO 11000 FT. THE CTLR REPLIED '11000 FT IS FINE.' AS WE THOUGHT ABOUT IT; NEITHER OF US HEARD A CLRNC TO 11000 FT AND WE REALIZED WE HAD 'BUSTED' THE 12000 FT CLRNC ISSUED EARLIER IN DSCNT. I TRY TO BE DILIGENT IN VERIFYING THE SELECTED ALTS ON THE FMP (AUTOPLT) AS PER SOP. I EITHER SLIPPED MENTALLY AND SELECTED 11000 FT OR SELECTED 12000 FT AND THEN BUMPED IT 1 CLICK TO 11000 FT AS I PULLED MY FINGERS OFF THE KNOB. I'VE DONE THIS BEFORE AND CAUGHT IT. IN EITHER CASE; I FAILED TO VERIFY THE ALT PROPERLY WITH THE PNF AS PER S0P NOR HE DID CATCH MY MISTAKE. FOR ME PERSONALLY; THE HIGHLY AUTOMATED A320/319 HAS BECOME A BATTLE AGAINST COMPLACENCY; ONE TO FIGHT EVERY DAY.