In development, I would normally have my own test scripts that would document the data, scenarios and execution steps that I plan to test; this is my dev test plan. When the functionality has been deployed to Test, testers test it using their own test script that they wrote. In UAT, the business user then tests using their own test plan.
In retrospect, it looks like this provides a better coverage, with dev tests having a mix of black and white box testing, while testers and business users focus on black box testing. But on the other hand, this brings up distinct test cases that only are executed per stage (ie. some cases which testers thought of are only executed on Test stage) and it would like the dev missed it, which makes it a finding/bug.
Is it worth consolidating the test scripts from the start? Thus using one unified test script, or is it abit difficult to do this upfront?