Anytime you work without being clocked in is a major violation of labor law, whether intentional or not.
Wage and hour division sets this general guideline
The amount employees should receive under the FLSA cannot be determined without knowing the number of hours worked also included is any additional time the employee is allowed (i.e., suffered or permitted) to work.
Employers are responsible for making sure employees are getting paid for their work. FLSA takes this offense very seriously, even if it was a mistake on the employee’s behalf.