Halacha depends on the facts of a situation. Generally, when one cannot know the exact facts, he is expected to assume that they are as usual (rov), or as they were when last checked (chazaka). When ...
The Gemara in Shabbos says that Bibi bar Abaye asks if one can violate a Rabbinic commandment (unsticking a loaf from an oven) to save oneself from being "chayav a chattas". The Gemara says that ...