After the wine steward is released from prison and returns to his job, he forgets to mention Yosef's name as a recommendation for release. After Pharoah has his dreams, the wine steward comes to Pharoah and tells him about Yosef and his ability to interpret dreams.
In 41:9 as he is telling Pharoah about Yosef he says, "I must now mention my sin" [in forgetting to mention Yosef].
Why does he call it a sin to Pharoah. As far as Pharoah is concerned he did nothign wrong. He could have just come to Pharoah and said I know someone in prison who has the skill of interpreting dreams... Yosef did not even really do anything for him - he just interpreted a dream. Even without his having been there, after three days the guy would have been released anyway. Yosef did not cause him to be released. So by forgetting to mention Yosef, he was not really sinning or doing anything wrong, so why did he say here that he must admit his sin?
The wine steward was probably living with a tremendous amount of guilt over the course of the 2 years that have passed since he was released from jail. Yosef, the guy who relieved his nerves by giving him a good interpretation asked for one small favor - just mention my name when you get out. He failed to do that simple favor.
It is even understandable. Upon his release, what should he have done - gone up to Pharoah and say I know another prisoner who I think you should release? That would have been fairly impudent of him, and I doubt he could have even said anything to Pharoah without getting his head cut off. So he kept it inside, even though he felt bad. Then as time went on, it kept nagging him in his head more and more. He kept thinking of Yosef to whom he had made a promise but did not keep. He felt very guilty.
Now an opportunity finally presents itself. Pharoah is looking for someone who can interpret dreams. He jumps at the opportunity and runs to tell Pharoah about Yosef, thereby relieving his guilt. He felt so guilty, in his own mind he had sinned to Yosef and that pushed him to make amends.
Guilt can be a big and powerful motivator.