[CrHn3]

It is my understanding that heating edible oils will not produce trans fats unless there is a catalyst to donate hydrogen, a vacuum and a prolonged period of time longer than average cooking times. It is unlikely that a person cooking at home will create trans fats.

Different oils have different smoke points, and heating an oil beyond that smoke point can polymerize the oil and produce free radicals.

Charring foods can create heterocyclic amines, which are associated with cancers.

[MorrisMinor]

Liquid oils contain cis double bonds that will isomerize to trans when heated (no H required). Short cooking times will minimise this, or you can use a saturated fat like butter.