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You can't really say "Task failed successfully" in Norwegian, my mother tongue.

You can say that a task "failed", "was successful" or "was accomplished", and even that it "failed in a successful way" but there is no adverb "successfully".

The word is "vellykket", well-lucked.

You could attempt to invent an adverb "vellykkelig" (well-luck-ly), but it doesn't exactly roll off the tongue, does it, and also, "lykkelig" means blissful or happy, so that alone breaks it.