observers_may_not_be_exclusive.md

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title: Exclusive systems may not be used as observers
pull_requests: [19033]

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Exclusive systems may no longer be used as observers. This was never sound, as the engine keeps references alive during observer invocation that would be invalidated by &mut World access, but was accidentally allowed. Instead of &mut World, use either DeferredWorld if you do not need structural changes, or Commands if you do.