Suppose one does an act on shabbas about which there is a machlokes whether it is forbidden d'oraysia or not (perhaps even a posek holds it is permitted). This person does the act regardless, but then ...
In my experience, there is a lot of emphasis on waiting until after Tzeith HaKochavim (the stars come out), which is seen as definitely after nightfall, to start the Seder. The reason is that we want ...
In this answer, I proposed that since there is a machlokes rishonim on a certain issue, it counts as a safek (doubt), although the Shulchan Aruch rules strictly. DoubleAA commented, saying that he ...