According to halacha, what is the earliest time (zemanim) one can accept Shabbat? Basically one can accept Shabbat before it would naturally start, but what is the maximum amount of time before Shabbat would naturally start that a Jew can accept it? If there are divergent views, what are they?
While it definitely seems that this is the prevalent opinion of poskim, historically there were communities in Europe that even accepted Shabbos at a much earlier time of day and that there were also some notable poskim who accepted this either on a בדיעבד or even a לכתחילה level.
I first remember hearing of such a community minhag to accept Shabbos very early from one of my grandfathers a"h (a Galitzianer who grew up not far from Lemberg/Lvov) and have since heard an additional actual case from another person.
If you search Google for קבלת שבת לפני פלג המנחה, you will get a number of relevant sources that discuss this topic including this very good shiur by Rav Asher Weiss (if you understand Hebrew and have an hour) who gives the topic a thorough treatment.