Thanks for pointing this out, because ESL readers as me, when shown an error repeatedly, start confounding the good version and the bad one. The worst part is that it is subtle: you understand what it's meant, noone mentions it, and something in your brain accepts it as a fact.

My fiancee is French and whenever she speaks (an almost perfect) Spanish, I always make the effort to repeat back errors she makes, in the correct form, without interrupting the dialog. On the medium term, that single act works wonders. I suggest that as a subtle alternative to pointing out errors, that can also be handy!