I've noticed recently that Memrise seems to think I am getting some words wrong in Hindi when they are correct. I'm pretty sure this is a recent development.
I think it is something to do with the order in which the letters are typed. For example, I was recently told I had got the word कल मिलेंगे (kal milenge, see you tomorrow) wrong. I think this may be because there are two ways to type the syllable लें (len): either ल + ं + े or ल + े + ं. I.e. the diacritics can be added in either order, but Memrise only accepts one as correct.
A similar thing happens with R's. An R before a consonant can be typed out as र + ् or using Alt+4 for the same result: र्. I think that, again, Memrise probably only accepts one of the two as correct.