Trans fats are a by-product of hydrogenation. If the reaction conditions are
rigorously controlled, they can keep the trans fats down to less than 1%.

During hydrogenation on an unsaturated fatty acid, an intermediate forms that has an
unsaturated (double) bond between two carbons open up to form two bonds with the
catalyst (often nickel, but sometimes platinum or palladium). Hydrogen gas adsorbed
to (on the surface of) the catalyst breaks apart to single atoms, and replaces the
bond with the catalyst an atom at a time. Sometimes, though, the fatty acid
temporarily bonded to the catalyst releases its hold without fully reacting with
hydrogen. That will reform a double bond, but it has a 50/50 chance of reforming in
the cis or trans conformation. The vegetable oils being saturated start out all
cis-bonds, but the hydrogenation process produces new trans bonds as an incidental
product of the intended reaction.

http://alfa.ist.utl.pt/~fidel/creac/sec35a2.html

Depending on the care used in processing, partially hydrogenated fats will have
perhaps a little, or perhaps a lot, of trans fats.

Lar