I've got power coming out of the negative side of my battery???

[HSkinner]

K' Just finishing up my truck and I was dialing in the carb when I went to crank it over and lost all power to everything. I checked for the obvious stuff and got the tester out and discovered that the entire block, frame etc. had power going through it. The positive side had nothing... I freaked and unhooked the battery and started checking for a grounded wire somewhere and found nothing. I never saw a spark of any kind. no burnt fuses. I waited a bit and reconnected the battery and it started right up and I havent had a problem since. What would cause this? It's a brand new battery from Wallys Wolrd and I'm nervous it could happen again. Any suggestions.