Besides what's already been mentioned, there are a few leniencies because of this, though I've never heard of your specific one.
There's a much larger discussion in halakhic literature about how careful (or not) we are about davening maariv at the proper time, and the Rambam (3:7) believes that this is because Maariv is a reshut.
The Beis Yosef (268:13) says that if one forget to mention Shabbos on Friday night davening, then it's enough to hear (or say with the chazzan, according to the Magen Avraham there s.k. 15) the 'Beracha Me'en Shalosh' because Maariv is a reshut.
Another nafka minah (practical difference): the Rambam (Hil. Tefillah 10:6) believes that if one suddenly remembered in the middle of his prayer, even in the middle of a beracha, that he had already davened shemoneh esreh, than he should stop right away. However, when it comes to Maariv, since he didn't technically need to daven anyway, he can continue praying despite the fact that he had already davened maariv. In practice, however, the Raavad (there) argues, and therefore the Aruch Hashulchan (107:6) states that one should add something to his prayer as per the general rule of 'tefilat nedava' (optional prayer). (However, this isn't true according to Reb Chaim on the Rambam there).
Another difference is that according to R' Yaakov of Lisa (Derech HaChaim) is that if one is unsure whether or not he has davened, he normally should daven (what may be a second time), but this is not the case for Maariv. However, the Aruch HaShulchan argues (107:10) so I wouldn't follow this lehalacha.