What does "Reviews Completed" mean one week post-submission? I sent a paper to a Springer journal. After two days the editor was assigned and after 5 days the status changed to "Reviews Completed".

Is this bad news?
Yes, it's a bad sign. If a review is completed that quickly the odds are the reviewer recommended a quick rejection. There's also a small chance the reviewer said something like "I can't understand this paper's English" in which case you might still receive a revise decision. However it's also possible that the editor invited multiple reviewers of which only the first has submitted, so it might be a while longer to a decision. If the other reviews are more positive, you might also get a revise decision.

Human error here is unlikely. The editorial management systems I've seen automatically update the status only when reviews are submitted. If the editor's making a decision without reviewers, the status never shows 'reviews complete'; it goes straight to 'decision in progress'.