What should be done with a glass showing rust at the cap?

When a glass shows rust at the cap, it raises a significant concern regarding the quality of the beer contained within it. Rust can indicate corrosion that might lead to metallic compounds leaching into the beer or could be a sign of poor sanitation practices. Checking for beer quality involves assessing whether the beer has been contaminated or compromised due to any interaction with the rust.

By prioritizing the check for beer quality, one ensures that they are maintaining high standards for serving beer to customers. It helps prevent serving a product that could affect the taste and safety of the beer, thus ensuring a positive experience for consumers. Discarding the glass or rinsing it might not address the fundamental issue of the potential quality of the beer inside, while serving it anyway could lead to negative repercussions for both the drinker and the establishment.