1) have a project with a shadow build directory where a build already exists and Qt Creator finds out that it is "incompatible" to the current build settings (you can e.g. achieve that by changing the compiler in the kit)

2) you'll have a warning in the project's build settings, below the shadow build directory setting

3) change the kit to match the existing build (e.g. by setting the correct compiler again)

4) the warning doesn't disappear

Also the other way round, the warning doesn't appear if you have a "matching" build and then change the kit