Even though we are determinedly engaged in devotional service, we may slip and fall, deviate, or commit a sinful act because as Srila Prabupada explains that we may still have a connection with our past sinful activities due to our long association with the material energy. Still, if we are rightly situated in our determination to worship Krishna purely, we will sincerely repent that sin and endeavor not to commit it again. Of course, part of our repentance will be our willingness to accept the reaction for the sin. Actually, it is this willingness to accept the reaction that distinguishes a sincere devotee from an insincere devotee: a sincere devotee will see the reaction as both deserved and a help to remove the sinful desire from the heart; an insincere person will actually want to commit the sin again but will not want to suffer the reactions.
(LFD, Vol2, Becoming Free from Sin, Pg 41 and Pg 42)