HaLeiVi – “Where do we find a Kabbala Baal Karcho?”
Examples:
See shitos that saying Yom Hashishi in Friday night ma’ariv is Kiddush, even though there is no intention.
Also see shitos about answering a friend questioning what is tonight’s sefira that if answered directly you cannot count that night with a brocha.
Regarding kabbalah b’tous (mistaken kabbolas shabbos), see halochos if one is mekabel shabbos because it is getting dark (thinking it is shekiah) and when the clouds dispersed he realized there is still much time before shkiyah. There needs to be an element of charotah.
Each example is a discussion of its own.
In any event, the OP asked “usually frummer folks — say Ah Gutten “Erev” Shabbos. Is there an inyan to not say Gut Shabbos before it is Shabbos?”
Apparently, you are noheg like the frummer folks who don’t want to get into a shaylah of an inadvertent kabbolas shabbos followed by issur melacha.