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Halacha applied differently dependent on the situation
I recall asking someone about whether a Jew could work in a non-kosher restaurant. One of the opinions I heard was that a Jew shouldn't because "odds are" that the person, as a waiter, will end up serving treif food to a Jewish customer. I don't know, then, if this would only apply in areas of the country/world in which there are Jews (or is the concern that someone who doesn't know he is Jewish will be served), or whether this is a lo plug case and it would apply even in a place in which it is known that there are no Jews.
Are there other instances in Judaism in which halacha is applied because "odds are" and which would be applied differently in other places/times?
I am looking for examples which are not related to questions of bittul (rov, shishim etc) of specific items of food and I'm not looking for case-by-case decisions that are dependent on financial loss.
edit------I found some notes I wrote to myself last week and they seem to be connected---------
If I recall correctly, originally, when people were called to the torah, each person called up read "his" section. So there had to have been a time when the expectation that all could read was grounded in a reality. This was changed to a single reader so as not to embarrass those who were not able to read. But that means that in an earlier time, there was an expectation that the average person could read (7 out of 10 every shabbat).
Is it reasonable to deduce that, during a certain time period (before there was enough decline in literacy rates to trigger change) that synagogues had to begin to be worried about embarrassing anyone – if the possibility that even a single person could be embarrassed was such a concern then the practice of expecting people to read their Aliyah would never have been established (it would be codifying a michshol for the gabbai).
Could the decision have been local? A village of tzaddikim b'nei tzaddikim would not develop this concern as quickly as a random village of lesser educated people.
Would it (the expectation that a person called up will read his aliyah) still be forbidden in a local minyan today made up only of professional ba'alei kriah?