Q. If someone did a demeritorious deed/unwholesome act thinking that s/he was doing a meritorious deed/wholesome act, what would be the true outcome? (Will s/he gain merits or demerits?). 

Please kindly advise with dhamma sources. Thank you very much. Theruwan saranayi! 

A. If s/he doesn’t know what exactly the meritorious or the demeritorious are, which means ignorance (moha), one of the demeritorious mental factors plays a huuuge role in the mind. And when s/he’s mistaken the meritorious as the demeritorious and vice versa, here comes false view (ditti / micca ditti - another demeritorious mental factor) into play together with ignorance; now guess what – s/he is entangled and in a biiig problem / trouble.