cache can get populated with pageNotFoundMessage, multiExcerptNotFoundMessage, etc.
The ME-Include method for getting excerpt body content can return error text for some conditions such as not being able to find the page that the excerpt is on.
The error text can end up in the ME-Include cache if the cache is empty when the message is generated.
Never cache any error text from the rendering of a ME-Include.
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fixed in 5.4.2