When running tests via AxUnit runner on the local PC and not on the PLC, AxUnit will compile a native executable that is run on your machine. This executable underlies all restrictions any other process on your system underlies. This means, if your program code under test contains any runtime errors like addressing out of array bounds or accessing NULL references, the process will crash.
Depending on your operating system, you may not get any feedback except that the test execution silently quits because the OS killed the test runner. In such cases starting the test execution in debug mode will very often reveal the error location as the debugger stops at the crash location.
For runtime errors on the PLC, the PLC will enter the STOP state and the error is added to the diagnostic buffer of the PLC.