"not Sub'a'pu' lulIjlu'."
Translation:Great heroes are never forgotten.
For this "Type what you hear" exercise, I wrote not Sub'a'pu' lulIjpu'. My mistake, I did not hear correctly (plus, I see now the grammar is wrong). However, Duo simply marked me correct without noting the error. It was only because the translation was fairly different from what I expected that I am aware that I made a mistake.
I don't think Duo is making this error only with this sentence. Marking "typos" right without correction seems to be global for this type of exercise in Klingon. If this is a known bug, I didn't notice it mentioned anywhere.
Long ago, the admins of Duolingo decided that a Yellow marking for "right but with a small error" was too negative. They mark that green now, but were still giving messages letting you know you had a typo (or a missing accent for courses that use those). However, it seems there was a fairly recent change that when there are multiple messages that the correction algorithm wants to give you, it prioritizes them and only gives you one message. So the "you had a typo" message is often not being shown. It seems like pretty big problem, but not one the contributors can do anything about. If it happens again, please take a screen shot and send it to https://support.duolingo.com/hc/en-us/articles/204728264-How-do-I-report-a-bug-