A BE20 INADVERTENTLY TURNED THE ALT ALERTER AND AFTER ENGAGING THE AUTOPLT CLBED TOWARD THE WRONG ALT UNTIL ATC NOTIFIED THEM OF THE ERROR.

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WE HAD JUST DEPARTED RWY 31 AT ZZZ1; AND WERE EXECUTING A R TURN OUT TO THE E TOWARDS ZZZ2. IT WAS TO BE A SHORT FLT OF ABOUT 13 MINS. ATC HAD CLRED US INITIALLY TO 7000 FT. THIS ALT WAS APPROPRIATELY ENTERED INTO THE FMS GUIDANCE SYS ALT PRE-SELECTOR PRIOR TO DEP; AND WAS CONFIRMED BY BOTH CREW MEMBERS. I WAS THE PF; AND USED THE FLT DIRECTOR TO HAND-FLY THE DEP. AT ABOUT 5000 FT I SELECTED AUTOPLT TO ON; AND NOTED THAT THE ALT IN THE PRE-SELECTOR SHOWED 8000 FT. I ASSUMED THE FO HAD RECEIVED CLRNC TO THAT ALT AND IT DID NOT REGISTER IMMEDIATELY THAT THIS ALT WAS DIFFERENT FROM OUR ORIGINAL CLRNC. ABEAM THE ARPT AND IN CRUISE-CLB THROUGH 6000 FT (APPROX) MY FO ANNOUNCED THAT HE WOULD BE OFF THE #1 COM RADIO WHILE OBTAINING THE ATIS FOR OUR DEST OF ZZZ2. I REPLIED THAT I HAD THE #1 COM AND POINTED OUT QUICKLY THAT I WAS IN THE CLB TO 8000 FT. HE DID NOT RESPOND AS HE TUNED IN THE SECOND RADIO. ATC QUERIED US AT 7600 FT AS TO WHAT WE WERE DOING; AND I REPLIED THAT WE WERE CLBING TO 8000 FT AS INSTRUCTED (OR SO I THOUGHT). THE CTLR GAVE US AN IMMEDIATE TURN AND DSCNT BACK TO 7000 FT. LESSON LEARNED: PAY CLOSER ATTN TO THE ALTS THAT YOU HAVE BEEN CLRED TO PREVIOUSLY; AND DON'T ASSUME THAT WHAT IS SIMPLY IN THE PRE-SELECTOR IS CORRECT. WE DETERMINED THAT ALTHOUGH IT IS UNCLR WHO TURNED THE ALT PRE-SELECTOR KNOB; IT IS LIKELY THAT ONE OF US DID SO INADVERTENTLY WHILE REACHING FOR THE HDG CTL KNOB LOCATED ADJACENT TO IT. ALSO; DON'T ASSUME THAT THE OTHER CREW MEMBER AGREES WITH YOUR STATEMENTS IF HE OR SHE IS SILENT.