Sydney Water completed UAT and parallel payroll before its Dayforce go-live. Employees still ended up with underpayments, overpayments and other pay issues.
In April, NSW Parliament was told testing was being used to identify configuration and Enterprise Agreement alignment issues before go-live. A few months later, employees were reporting pay and roster problems.
What caused that is not public, so I won't speculate.
The challenge that arises is a practical one for any payroll program approaching cutover: what evidence are you using to decide payroll is ready?
A completed test plan tells you the planned testing was completed. It does not tell you whether the scenarios were representative of your real population, whether the results were reliable enough to trust, or whether the remaining defects and exceptions were understood well enough to support the decision.
The test plan records what you ran. The readiness decision rests on what you concluded from it.