"I forgot just one time."
Translation:저는 딱 한 번만 잊어버렸어요.
While the supplied translation is correct, it seems that there are various ways to say this, especially given that the first two words in the correct response can be seen as implied, in the Korean language. (Imho, as a native speaker who is trying to use this app to improve her written skills in the language...)
As a native speaker, feel free to report/flag any missing translations you can think of. This will help them get out of beta faster.