ALTDEV. WRONG ALTIMETER SETTING IN ALERT WINDOW.

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SWITCHING FROM 1 CTLR TO ANOTHER IN ZNY; I RPTED LEAVING 16000 FT FOR 18000 FT. AT 17000 FT THE FO ASKED IF WE WERE CLRED TO 17000 FT. 18000 FT WAS IN THE ALT WINDOW SO I TOLD HIM WE WERE CLRED TO 18000 FT. ABOUT 30 SECONDS AFTER LEVELOFF; ATC ASKED US OUR ALT AND I SAID 18000 FT. HE SAID WE WERE ONLY CLRED TO 17000 FT. I REMINDED HIM THAT I CALLED OUT OF 16000 FT FOR 18000 FT AND HE ACKNOWLEDGED. HE THEN SAID MAINTAIN 18000 FT. NO OTHER PROBS OR CONFLICT. THE FO HAD A HABIT OF RESTING HIS FINGERS ON THE GLARE SHIELD ABOVE THE ALT SELECTOR KNOB. MY GUESS IS HE HIT THE KNOB AS HE MOVED HIS HAND FROM THE GLARE SHIELD TO THE YOKE. THAT WAS HOW 18000 FT ENDED UP IN THE WINDOW. I REMINDED HIM OF HIS BAD HABIT OF PUTTING HIS HAND ABOVE THE ALT CTL KNOB. THE AMBER 'ALT ARM' LIGHT WAS ON IN OUR ANNUNCIATOR SO THE KNOB MUST HAVE BEEN PULLED. WE PROCEEDED TO CLE WITHOUT FURTHER INCIDENT. WE WERE USING THE 2 PLT SYS OF SETTING ALT; THAT IS; I CALLED OUT THE ALT AND HE REPEATED THE ALT AND SET IT IN THE ALT WINDOW SINCE HE WAS FLYING ON HIS AUTOPLT. OTHER THAN KEEPING HIS HANDS FROM LINGERING NEAR THE ALT KNOB; I DON'T KNOW WHAT ELSE COULD HAVE BEEN DONE. ATC WAS NOT A PROB. THE CTLR DID HIS JOB WELL.