ALT OVERSHOOT OVER DKK VOR.

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OUR FLT WAS CLRED TO CROSS DUNKIRK VOR (DKK) AT 11000 FT; 250 KTS. FO PF PLACED 11000 FT IN ALT WINDOW AND CAPT (ME) PNF POINTED AND VERIFIED 11000 FT. WX AT BUF WAS IFR WITH LOW CEILINGS; LOW VISIBILITY; SNOW; VERY STRONG SURFACE WINDS; MODERATE ICING; AND TURB IN CLOUDS. WE WERE STILL ON TOP IN THE CLR. WE BEGAN THE DSCNT FOR 11000 FT; BRIEFED THE APCH; RAN PRELIMINARY LNDG CHKLIST; AND MADE A PA TO PAX. WE SET FLT ATTITUDE DOWN FOR UPCOMING TURB IN DSCNT. WITH ALL ITEMS COMPLETED; FELT THIS WOULD BE A GOOD OPPORTUNITY TO QUICKLY USE THE LAVATORY BEFORE ENTERING THE CLOUDS AND TURB. UPON RETURN TO FLT DECK; NOTICED 10000 FT IN ALT WINDOW. FO (PF) DSNDING THROUGH 10800 FT FOR 10000 FT. I ASKED FO 'DID THEY GIVE US 10000 FT?' RESPONSE 'YES; 10000 FT.' I ASKED AGAIN; 'DID ATC GIVE YOU 10000 FT?' RESPONSE 'YES; 10000 FT.' NOW PASSING THROUGH 10500 FT AND LEVELING BEFORE FURTHER DSCNT; ATC ASKED US 'WHAT WAS YOUR ASSIGNED ALT?' BASED ON SEEING AND CONFIRMING 10000 FT WITH FO; REPLIED '10000 FT.' ATC REPLIED 'I'LL GET YOU CLRNC TO 10000 FT.' (AT TIME NO TFC OBSERVED ON TCASII; 20N MI SCALE AND BELOW.) ATC SAID 'MAINTAIN 10000 FT.' I ASKED ATC 'ANY PROB SIR?' RESPONSE 'NO; NOT YET;' AND THEY HANDED ME OFF TO BUF APCH. UPON LNDG BUF; I CALLED ZOB AND SPOKE WITH A MGR WHO SPOKE WITH THE CTLR. SAID 'DON'T WORRY; THERE IS NO PROB; SIR.' THANKED HIM. I DON'T KNOW HOW 10000 FT WAS SELECTED INTO THE ALT WINDOW WHILE I WAS OUT OF FLT DECK. I KNOW I SAW AND VERIFIED 11000 FT BEFORE I LEFT. MY FO WAS FAIRLY NEW -- JUST OVER 1 YR. POSSIBLY COULD HAVE BUMPED KNOB WITH HIS HAND OR WAS BUSY MAKING FMC ENTRIES OR SECOND-GUESSED THINKING IT WAS 10000 FT. I DON'T KNOW. EITHER WAY; SAME STORY. ALWAYS VERIFY AND IF NOT SURE; SIMPLY ASK ATC. BUT BASED ON SEEING AND VERIFYING WITH FO 'DID THEY GIVE US 10000 FT?'' TWICE AND ANSWER WAS YES; I BELIEVED THEY GAVE HIM 10000 FT.