I've been thinking about this for a while, and this question reminded me.
If you're walking past a shul between plag and shkiya on Friday Asarah Beteiveit and you hear Barechu but don't know if it belongs to mincha or maariv, and you are not ready to accept Shabbos yet, can you answer?
It's hard to imagine that it's possible to not know whether with a particular action you are accepting Shabbos or not - either you have daat or you don't. I understand that in an ordinary situation responding to barechu automatically means you have daat that it's night=Shabbos, but here you don't know whether this barechu belongs to the night or the day.
(This could really be relevant in a place where some people follow the more lenient psak as mentioned here and are therefore davening early, assuming that the baal tefilla is inexperienced and can't do the Friday night niggun for barechu. I've heard some who can't.)