Directors of corporations can be required to pay source deductions which are actually owed by the corporation. Obviously this is not desirable and only occurs when the corporation is not able to pay their own bills. When a director is assessed by CRA to pay a corporate debt personally, it is referred to as derivative tax liability. There is a two year limitation period however.
A director must be assessed by CRA within two years of the date when they ceased being a director. Sometimes a director resigns in order to start the clock ticking on that two year assessment period. However, if a director continues acting as a director, even though they have resigned, the fact that they have resigned is not relevant.
A director who is appointed is a De Jure director. A director who acts as a director is a De Facto director and CRA will only consider the two year period to have expired when a person is neither a De facto nor a De Jure director. If you resign as a director you should also stop fulfilling the duties of a director.