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According to TheBottleGuide.com
What does paneling refer to in plastic jars & bottles?
Paneling refers to the deteroration and eventual collapse of the walls on a plastic container. It will usually occur when a bottle or jar (with specific contents) has been stored for a long period of time. This type of distortion is cause by the slow reduction in pressure inside the bottle or jar. Paneling will normally occur to a container with a less rigid plastic such as LDPE. In short, paneling is the slow, inexorable and impending implosion of your bottle.