The sign convention for the Pfaffian of a skew-symmetric matrix on the Wikipedia looks like the negative of the sign convention in Lang's Algebra (at least looking at the 4 by 4 matrix in the Examples section):
http://en.wikipedia.org/wiki/Pfaffian
Obviously if we were starting mathematics from scratch there would be no harm in defining it either way, but I wonder if one way is more standard. (In the application I have in mind, Lang's way is better.)