Incorrect word strikethrough bug
If you make a mistake in a single word in a sentence, Duolingo will colour the mistaken word red and strike it through. However, it seems to calculate the offset of the strikethrough based on the beginning of the sentence after padding has been removed. This means that if you have extra whitespace at the beginning of a sentence, the wrong text range will be modified.
Here's an example (in which the word "better" should be highlighted):