Exercise 3: Launching With Test Mode Still On — Possible Solution
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WHAT TEST MODE ACTUALLY DOES
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Per this chapter, Test mode swaps a gateway's live API keys for its
sandbox environment - the same environment used with dedicated test
card numbers (like Stripe's 4242 4242 4242 4242) to simulate a
checkout with no real money ever moving.

WHAT A REAL CUSTOMER WOULD EXPERIENCE
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Per this chapter, if Test mode is left enabled after launch, real
customers' checkout attempts silently fail to charge anything - the
chapter states directly that in this situation "the store looks
broken to a paying customer." A real customer entering their genuine
card details would go through the checkout flow, but the transaction
would be processed against Stripe's sandbox rather than its live
payment network, so no real charge would actually succeed, likely
producing an error or a payment that never completes.

WHY THIS WORKS AS AN ANSWER
------------------------------
It correctly explains that Test mode substitutes sandbox credentials
for live ones, and correctly describes the customer-facing symptom
this chapter names directly: checkout appears broken, since real
payments cannot succeed against a sandbox environment.