In Mas. Kiddushin, 45b, the gemara discusses a woman who has undergone eirusin but the status of her nesuin is in doubt. The question arises as to whether she can (if she married a kohein) eat of his t'rumah. One element of the answer is that, min hatorah, she can because the eirusin status is enough to allow her to do so regardless of the nesuin status, but the robonon extended the law and require nisuin so as to avoid potential problems that could arise if she, still living in her father's house, brought back any t'rumah from her betrothed. [from note 10 on 45b2 in the Schottenstein, which cites Ketubot 58a-b].
There are other cases where, in order to forestall potential problems, a fence is made around what is min hatorah, allowed, and the practice is to follow the derobonon's forbidding of it.
Is there a master list of such rabbinical enactments forbidding what was mide'oraita permitted? There are, in the gemara, practical consequences, or at least, theoretical halachic implications to these distinctions (see note 14 on 45b2) so a complete list would be helpful in parsing certain halachic arguments.