switched neutral is possibly why the white wire was hot. but just reversing the polarity shouldnt trip the breaker unless there is a short to ground downstream of the panel. Is there a tool that will locate a short, Ie inspector 2 or 3 and measure leakage? this house had some remuddling done before I arrived and just added new circuits as needed. otherwise my knowledge would tell me that somewhere there has to be a grounded box with the return wire bonded to it to make a dead short when polarity was switched, or would a possible old three wire k t circuit trip a breaker if one phase was swapped with the neut.? previous owners had new K&T wires pulled to old fpe panel for new kitchen remodel, prior to 1990, and a new 200amp panel for new remodel circuits, but kept the K&T, I've bypassed the fpe but trying to ring out the old circuits has been trying.