Before
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If you load a new version of the CiviCRM codebase, then a freshness check
should cause the container cache to reset automatically (based on the fact
that various files have new timestamps).

However, it's possible that some kind of bug or omission prevents this from working.
Many developers won't notice such a bug because they're obsessive-compulsive
about clearing caches anyway.

After
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If you load a new version of the CiviCRM codebase, then it should use a new
container cache - regardless of how well the freshness check works.