In the spirit that one of the more valuable functions of the LML is to pass along experiences that should be avoided by fellow builders, I need to share a situation that could have been very bad and could happen to anyone. Recently we installed a new instrument panel in a Legacy and were going thr ough all the various systems checks to assure they were functioning correctly.  When it was time to check the hydraulic pump function we made sure that the gear selector switch was in the down position and when the circuit breakers for the relays and pump were pushed in the gear retracted!  Fortunately the plane was up on jacks at the time or this could have been a serious matter.  It turned out that the up and down wires were reversed on the gear selector switch, which is not hard to do as the "up" wire is on the bottom terminal and the "down" is on the top.  This also bypasses the gear safety switch.  Yes, this panel allegedly went through a multi-day check out period before we got it.  Bottom line:  keep the plane on jacks during the entire initial panel check out.  You ES and FG guys excepted, of course.  Just thought this was worth passing along.  Leighton Mangels