FLT CREW OF MD80 INADVERTENTLY CLBED TO 14000 FT WHEN ONLY CLRED TO 13000 FT.

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ALT WAS INCORRECTLY SET IN ALT CAPTURE WINDOW OR CHANGE KNOB WAS BUMPED TO DIFFERENT ALT (WHEN AUTOPLT WAS ENGAGED). ASSIGNED ALT WAS 13000 FT AND AN ACFT WENT THROUGH 13000 FT BOTH PLTS POINTED TO ALT SET KNOB AND RESPONDED 13000 FT FOR 14000 FT. SHORTLY THEREAFTER; CAPT STATED 'WHEN WERE WE CLRED TO 14000 FT?' (NOTE THAT 14000 FT WAS SET IN.) AS WE WERE LEVELING OFF AT 14000 FT; CTLR INQUIRED ABOUT WHAT ALT WE WERE AT AND CAPT STATED '14000 FT.' AT THAT TIME; CTLR TOLD US WE WERE ONLY CLRED TO 13000 FT. SECONDS LATER; ATC CLRED US TO FL230. NO CONFLICT OCCURRED. AFTER DISCUSSING SIT WITH CAPT; HE STATED THAT HE WAS SURE HE SET IN 13000 FT. I AM PRETTY SURE THAT 13000 FT WAS SET IN WHEN CHANGE WAS MADE (8000 FT TO 13000 FT; AUTOPLT NOT ENGAGED). SOMEWHERE IN THERE; ALT IN READOUT WINDOW WAS CHANGED TO 14000 FT. WAS IT SET IN INCORRECTLY OR WAS IT BUMPED? WE'RE NOT SURE HOW THE CHANGE OCCURRED. ONCE ON THE GND IN DCA; WE EXPERIMENTED WITH THE SET KNOB TO SEE IF WE COULD SET AN ALT WITH THE SET KNOB (QUIT) NOT IN THE DETENT. (IE; 13000 FT SHOWING IN THE WINDOW AND THE SET KNOB NOT IN THE DETENT.) WE WERE ABLE TO DO SO AND WHEN I POINTED TO SET KNOB AND TOUCHED IT; I WAS ABLE TO CHANGE ALT TO 14000 FT. OTHER THAN SETTING ALT INCORRECTLY; THE ONLY OTHER SIT THAT WE THOUGHT MIGHT HAVE HAPPENED WAS THAT AS GOING THROUGH APPROX 11000 FT; I ENGAGED THE AUTOPLT AND MIGHT HAVE BUMPED ALT SET KNOB WHILE DOING SO. TO AVOID THIS SIT IN THE FUTURE; WE MUST BE MORE CAREFUL SETTING ALT (IE; WIGGLE KNOB SLIGHTLY TO MAKE SURE IT IS IN DETENT) IF A CHANGE IS MADE OR BE CAREFUL WHEN ADJACENT SWITCHES/KNOBS ARE OPERATED.