"When" is exactly one call

Make sure that the “when” section of your test method contains exactly one call to the class under test:

METHOD rejects_invalid_input.
  " when
  DATA(is_valid) = cut->is_valid_input( 'SOME_RANDOM_ENTRY' ).
  " then
  cl_abap_unit_assert=>assert_false( is_valid ).
ENDMETHOD.

Calling multiple things indicates that the method has no clear focus and tests too much. This makes it harder to find the cause when the test fails: was it the first, second, or third call that caused the failure? It also confuses the reader because he is not sure what the exact feature under test is.