A person davened Mincha Gedola, forgot that he had done so, and came to shul to daven Mincha 10 minutes before shkia. He finished davening. May one consider this tefilla to be Maariv, or do we say that he made 19 blessings in vain, and is obligated to daven Maariv?
This is an(other) excellent question. Most poskim that I discussed this were of the opinion that since he had intention for the wrong teffiloh he has to daven again. However in the final analysis, since it is question able he should daven again and stipulate that if he is actually required to daven maariv then this is what he is davening now, but if it considered as if he actually did daven maariv then he wants this tefiloh to be a tefilas nedava, (voluntary teffiloh).
When davening a tefilas nedava one has to concentrate on everything that he is saying throughout the whole SE.
Magen Avrohom 268-9 and Pri Megadim ibid, B’tzel Hachochma 5- 22, O”CH 107- 4, see Seif 1 that he doesn’t have to add something new (a personal request) since the fact that he has to daven is considered something new.