Tags can be mutated in most (not all) container registries. A tag that you have vetted today could change without your knowledge.
Helm chart versions can also be overwritten.
Ideally, everything should be pulled from repositories you control.
Which reminds me: never don't do that. Always use fully-digested references. Don't fail to use fully-digested references.
The relevant section is here: https://livebook.manning.com/book/knative-in-action/chapter-...
The gist is that tags are mutable and deletable. Local caching means that every node can disagree on what the tag resolves to, and so can the registry. What version corresponds to which tag at which location at a given time is not deterministic.
The only way out is to use fully-digested references. The digest means that there can be no mutation of what's being pointed to. It will mean the same in every configuration and on every node, no matter when the reference is resolved and no matter when the reference was created.