Is this against the rules??

[VaCoach]

After a time out, both teams have 5 players on the court and Team A inbounds the ball and is attacking there basket. All of a sudden a player on Team B leaves the court and returns to the bench and sits down, leaving Team B with only 4 players. (Reason why was later explained by the coach, she was to be replaced during the time out and when she realized she had gone back in, she thought they might have 6 players on the court and she thought she better leave before being detected.) Question: Is this a violation of any type? If so, what is the penalty?