Thirty seconds

The mistake

A session idle-timeout field is configured in seconds but the admin enters the value intending minutes, setting every customer session to expire after 30 seconds instead of 30 minutes. This is deployed store-wide with no staging step or confirmation dialog, so the change goes live immediately and starts dropping sessions mid-checkout. The same timeout applies to the admin's own login, so the settings page logs her out before each save completes, making the fix as unreachable as the bug.