What indicates that a beer bottle may contain old, unstable beer when examining it?

The presence of white flakes in the bottle is an important indicator that the beer may be old or unstable. These flakes typically result from protein and yeast sediment that can clump together, often due to age or improper storage conditions. In many cases, when beer is past its prime, or if it has been subjected to temperature fluctuations, these particles can become more prominent, suggesting that the beer may not be at its best quality.

While bubbles in the liquid can indicate carbonation, they don't necessarily signify instability. Dark coloration of the beer can vary widely depending on the style and doesn't fundamentally indicate whether a beer is old or unstable. Excessive foam during pouring can be the result of a variety of factors, including how the beer is poured or the carbonation level, but again doesn't necessarily point to the beer's age or stability. Thus, the presence of white flakes is a clear red flag regarding the beer's integrity and quality.