CTLR AND PLTS RPT A KC10 OVERSHOOT OF ALT NEAR ACY; NJ.

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ACT X; A DC10; DEPARTED WRI AND CLBED TO 12000 FT. ACFT Y; A J541; LEVEL AT 13000 FT; XING ACFT X. TFC WAS CALLED TO ACFT X AND ADVISED TO MAINTAIN 12000 FT. SUBSEQUENTLY; ACFT X; AFTER ADVISING ATC THAT HE HAD TFC IN SIGHT; CLBED TO 12500 FT INADVERTENTLY. ATC ADVISED ACFT X TO 'MAINTAIN 12000 FT.' ACFT X ROGERED AND ADVISED AUTOPLT WAS ACTING UP. BOTH ACFT PROCEEDED WITHOUT INCIDENT. SUPPLEMENTAL INFO FROM ACN 464083: WE WERE CLRED TO 12000 FT WITH OPPOSITE DIRECTION TFC AT 13000 FT. COPLT WAS FLYING ACFT. I WATCHED THE FMA GO FROM 'VERT SPD' TO 'ALT CAPTURE' THEN TO 'ALT HOLD' AT 12000 FT. I DIVERTED MY ATTN FROM THE ALT TO CHK SOMETHING ON THE FLT PLAN AND NAV CHART. A FEW SECONDS LATER I SAW THE ALT WARNING LIGHT FLASHING. I LOOKED AT THE ALT AND NOTICED US CLBING THROUGH 12300 FT APCHING 12400 FT. I IMMEDIATELY USED THE VERT SPD KNOB TO INITIATE A RAPID DSCNT BACK TO 12000 FT. THE HIGHEST WE GOT WAS BTWN 12400 FT AND 12500 FT. WE LEVELED OFF AT 12000 FT FOR THE SECOND TIME AND HAD NO OTHER PROBS FOR THE REMAINDER OF THE FLT. I DO NOT KNOW WHAT CAUSED THE ACFT TO START THE CLB OUT OF 12000 FT. I DID NOT SEE THE COPLT INITIATE THE CLB AND CAN'T SAY HE DID. SUPPLEMENTAL INFO FROM ACN 464082: THE OTHER PLT RECALLS SEEING THE ALT HOLD FUNCTION OF THE AUTOPLT ANNUNCIATED TO CAPTURE 12000 FT AS I DO. BECAUSE THERE WERE NO OTHER ALT CAPTURE DEVS DURING THE ENTIRE 4.7 HR SORTIE; I SUSPECT INADVERTENT BUMP OF THE VERT SPD WHEEL CAUSED THE ALT HOLD FUNCTION TO DISENGAGE.