Rav Aharon of Belz ztz"l was very makpid not to say or hear a bad word about Jews. His Gabbai once suggested that they go to Chaifa. Reb Aharon said that we can't go there because there are people there who think that Shabbos is Friday [instead of saying that they are מחלל שבת].
The Mishna says "כלל גדול בשבת השוכח עיקר שבת" which Reb Aharon explained to mean that when a Jew is מחלל שבת the rule is that one must assume that he forgot that it is Shabbos.
