While topping off one liquor bottle with the remnants of another behind the bar may look like a marriage of convenience, experts say the practice is anything but harmless and that customers should know the warning signs."Marrying" liquor bottles, as its known in the industry, is prohibited under federal law and state liquor laws generally mirror the rule. And the rule isn't just in place to stop bartenders from pouring the cheap stuff into top-shelf bottles: You can't mix the same brand, either."Under the rules of the Alcohol and Tobacco Tax and Trade Bureau, once a distilled spirits bottle is opened, it cannot be refilled or combined with any other bottle," said Sanwar Mal Khokhar, a mixologist and bar manager at Sanjh, a high-end Indian restaurant in Texas.BARTENDERS SAY ONE COMMON REQUEST COSTS THEM TIPS AND SOME CUSTOMERS HAVE NO IDEA"It is considered misbranding or tampering with a federally labeled product," Khokhar told Fox News Digital.The federal regulation, 27 CFR 31.201, prohibits placing any distilled spirits into a liquor bottle other than what was originally inside at bottling. It also bars adding any substance that alters the original contents. Violations can trigger fines, suspension or even revocation of a liquor license."Because a liquor license is essential to running an establishment, most professionals avoid doing this," Khokhar said.GEN Z REJECTS TRENDY NIGHTLIFE, FLOCKS TO THEIR OWN CHEERS FOR CHEAP DRINKS AND REAL CONNECTIONYet industry chatter suggests the practice isnt unheard of.In a Reddit thread in the r/bartenders community, dozens of bartenders acknowledged that while marrying bottles is illegal, it often happens out of sight and with well or rail spirits. Some described consolidating partial bottles at closing time to save space or streamline inventory."Its always been illegal, and weve always done it anyway," one Redditor commented.FLUFFY JUICE IS THE LATEST COCKTAIL CRAZE AND YOU CAN MAKE IT AT HOME"Its illegal in my state, but every bar Ive worked in does it," said another.Others pushed back, calling the practice unnecessary and a sign of poor bottle management. Yet some admitted to refilling premium bottles with lower-cost liquor, which is the kind of deception federal and state laws are designed to prevent."In the U.S., alcohol distribution is tightly regulated for tax control and consumer protection," Khokhar explained. "Refilling or combining bottles interferes with the system."CLICK HERE TO SIGN UP FOR OUR LIFESTYLE NEWSLETTEREach bottle is taxed and tracked, so if they are combined, batch identification is lost, which can create problems in the case of a recall."The major risk is contamination and loss of traceability," Khokhar said. "Once bottles are combined, you lose clear batch identification. If there is ever a recall or contamination issue, it is almost impossible to trace the source."There are also health concerns. Combining bottles increases the risk of foreign particles entering the container, improper handling and potential cross-contamination with allergens, he noted.CLICK HERE FOR MORE LIFESTYLE STORIESConsumer safety advocates have also warned that refilling bottles can open the door to fraud, including watering down liquor or substituting cheaper spirits for premium brands. Counterfeit alcohol can even lead to severe illness.While regulators and liquor control boards conduct inspections, customers can also spot subtle clues."The first sign could be mismatched caps or seals, labels that appear tampered with, unusual fill levels at odd times of service, or staff taking bottles out of guests sight and returning them or inconsistent taste between pours of the same brand," Khokhar said.Worn or peeling labels on bottles can also be a potential red flag, according to reports.In a well-run bar, the protocol is simple."Once the bottle is finished or empty, the first thing you need to do is scratch off the sticker and discard it immediately," Khokhar said.TEST YOURSELF WITH OUR LATEST LIFESTYLE QUIZIf a customer believes theyve witnessed refilling, Khokhar advises raising the concern with management first. If the issue remains unresolved, they can contact their state alcohol regulatory authority.