What can cause an offspring to have a physical trait neither of its parents has?

The main way that an offspring can exhibit a different physical trait than its parents is due to the fact of genetic mutation.

Explanation: A mutation within a parents sex cells can cause different traits to be shown in the offspring because these genetic mutations cause DNA to form that has crucial differences to the original DNA composition which can manifest itself as differences in physical characteristics. These can be not so serious mutations (such as someone being born with an extra finger) or they can be more serious such as leg or arm mutations which can mean arms can be crooked or shorter than they are supposed to be. I hope this has been of assistance to you and if you have any further questions please do not hesitate to ask :)