As u know, IIIrd law of motion says that for every action(force), there is equal and opposite reaction.
Now let us understand how and when Newton's IIIrd Law can be voilated:
Suppose two bodies "A" and "B" come in contact with each other.
In that case, the contact will lead to transfer of force from A to B (or vice versa).
Now, there is a subtle point here. Every force takes some time to get transmitted. So, if
by any chance, A and B remain in contact with each other for a time which is less than
the time for the force to get transmitted between A and B, it essentially means there is
no reaction, though both the bodies came in contact with each other.
This is a violation of Newton;s IIIrd law of motion..
For details, please see Berkeleys Physics book.