I believe that the TDML Runner used to complain if you attempted to set a negative test (one that expected and resulted in errors) to roundTrip="true". This is no longer the case. There are several test cases that are currently set to roundTrip = "true" that will never reach the second leg of the trip. They currently pass without any error or warning to the user that the test is invalid. For an example, see test infInvalidType in the following files:

daffodil-test/src/test/resources/edu/illinois/ncsa/daffodil/section13/text_number_props/TextNumberProps.tdml
daffodil-test/src/test/scala/edu/illinois/ncsa/daffodil/section13/text_number_props/TestTextNumberProps.scala