Both spellings are acceptable, although the scripts spelled it as felgercarb. Susan Paxton's Concordance (which heavily relied on the script spellings) uses felgercarb.
My guess is that the actor's sometimes found the alternate spelling easier to to pronounce. Fans sometimes used this spelling as well. Example: Feldercarb was the name of a fan magazine by Laura Michaels.
I don't consider feldercarb "wrong" - simply an alternate variant.
The Second Season bible is authentic as well as the Writer's Guide. The source of the second season bible is Chris Larson. When I finally found out it was the real deal, I made this comment at Cylon Alliance: "Thank God Battlestar Galactica was cancelled."
Sheba's Galaxy is a great source.
Here's two other favorite links of mine:
http://www.kobol.com/archives/index.html
http://www.geocities.com/sjpaxton/concord.html
