1. When it is your fault. This is the case where you were not מפריש and you will cause others to eat tevel so we allow you to violate an issur derabanan to be מפריש שלא מן המוקף. This can be explained as a problem of you doing an aveira of לפני עוור and thus not done for your friend but for yourself.
2. If it is a מצוה רבה. The classic example is freeing a half slave (an איסור עשה) to allow him to be fulfill פריה ורביה.
3. If the person did the aveira המזיד then we do not sin for him but בשוגג we would.
Based on 2 (and 3 above), the Shulchan Aruch סימן שו סעיף יד paskens that we DO violate an issur derabanan (and perhaps even safeik deoraisa -- see Mishna Berura about שלש פרסאות) for a grave aveira, i.e to save from shemad. The case he discusses is to violate techum shabbas to save your daughter (MB says not necessarily your own daughter) if she was kidnapped to convert her. Mishna Berura says that if she left במזיד to convert, then you still may be allowed to violate a pure איסור דרבנן. This is davka because it is not a one time aveira but will be a permanent situation (see taz).