Climbing through FL200 for FL210 with the autopilot engaged, we received an altitude alert indicating that we were 1000’ away from our level off. This was audibly acknowledged in the cockpit by both the PM and PF. At this point it is my belief that there was movement of the speed bug knob or heading bug knob which has a similar tactile feel and appearance as the altitude selector knob in this model of Falcon jet. This resulted in disabling the automatic altitude level-off function of the autopilot; upon realization of the altitude error, immediate corrective action was taken by the PF and a vector was given by ATC.
Apologies were made to ATC for the error. During the post flight debrief we discussed maintaining extra vigilance that the autopilot levels off at the correct altitude and that when changes are made moving flight guidance panel knobs, there is a corresponding indication on the Primary Flight Display.
