What are the exceptions to Lavoisier's Law of Conservation of Mass?

How would one go about modifying experimental procedures to obtain results that apply to the law of conservation of mass?

Thank you!

The law of conservation of mass is only approximate.

In any reaction, if you measure the masses before and after, you will find a very small difference in mass. This is due to the change in binding energy of the system, this is a relativistic effect.

As well as not applying in relativity, conservation of mass does not hold quantum mechanically either. Especially in pair production where a photon with a zero rest mass creates 2 particles with a positive mass.