It is Friday Night. The shul is due to daven Mincha at 7.30pm, but Plag Hamincha is 7.37pm. The Shul is two short of a minyan and the decision is taken to daven Mincha privately. It is now 7.45pm (and so past Plag Hamincha). After which, two more arrive and a minyan is then formed for Mincha. After davenning Mincha, they then decide to daven Maariv even though it is still not sunset let alone nightfall. Were they in error?
Don't forget to CYLOR, but the Ktzos Hashulchan (Siman 77 in the Badei Hahulchan sif katan 17 and 18) writes that one should pray Maariv earlier on Shabbos night, but one is not allowed to pray before Tzeis if he davened Mincha after Plag. However, if one did, he is Yotzei Maariv. (He must still say Shema again after Tzeis)