Temporary workers have a bone to pick with the world’s largest retailer.

Yesterday 20 temps filed a proposed class action in Chicago in which they claim Wal-Mart Stores Inc. and two staffing agencies—Labor Ready and QPS Employment Group—have been violating federal and state minimum wage and overtime laws since 2009 by requiring them to show up early for work, stay late and work through lunch without compensation. The plaintiffs seek back pay and an injunction against the three companies. The proposed class action could affect hundreds of Wal-Mart temps in the Chicago area.

To make matters worse, Wal-Mart could face a labor shortage just in time for the holiday shopping season. Workers from 28 Wal-Mart locations nationwide went on strike earlier this month to protest the company’s efforts to “silence and retaliate against workers for speaking out for improvements on the job,” according to a United Food and Commercial Workers press release. And according to Thomson Reuters, Wal-Mart workers across the country have said they plan to walk off the job on Black Friday in protest of purportedly tough working conditions.

For more recent Wal-Mart stories, read:

Ex-Wal-Mart employee fired for medical marijuana use loses appeal

Wal-Mart gender discrimination case will proceed

University of California hits Facebook, Wal-Mart and Disney with patent suit

SEC asks Wal-Mart for more details in its financial filings

Wal-Mart accused of violating ADA

Wal-Mart disapproves of $7.25 billion credit card fee settlement

Delaware judge reprimands two plaintiffs in Wal-Mart bribery case