Budweiser -- Investigating Bad Beer Allegations ... At NFL Stadium

The people at Budweiser tell TMZ Sports ... the company has launched an investigation to get to the bottom of why SKUNKED beer was allegedly being sold at the stadium where the Redskins play.

The pics went viral earlier today -- showing fans at FedEx Field in Maryland holding Bud Lights with the World Cup logo on the bottle that had CLEARLY passed their freshness dates.

And how do we know? Because we typed the bottle code into the "Track Your Bud" feature on Bud's website ... and it told us the beer had gone bad.

But Bud is taking action -- at least according to Pete Kraemer, Anheuser-Busch's Vice President of Brewing and Supply.

"We work closely with our 600-plus wholesaler network to ensure beer is handled and distributed to our product quality standards ... We are working closely with the wholesaler and retailer regarding this instance."

The good news -- when we entered the old beer's code on the Bud website, it told us the company wanted to buy us a free beer.

Good thing we're over 21.