The two most common errors when calculating spokes are,

Wrong ERD, either bad spec data, or measuring yourself and forgetting to add for the nipple heads.
Cross mismatch, calculate for 3x build 2x or a comparable error.

Another error arises from getting the wrong spokes delivered and not checking.

After those two, anything is possible. Recheck all measurements and the math to see where you might have gone wrong.

Also recheck that you're counting crosses correctly. Sometimes people err, trying to cross one more spoke than intended because they didn't count the cross right at the flange.