When replying to a discussion message (source mode) on my Android phone I sometimes press enter, accidentally, which I don't notice until a few words later. So I go back to delete that new line, and get some weird things: in the source input field everything looks normal, but not in the saved text.
Here's how to reproduce:

Enter some text [PRESS ENTER, accidentally]
continue writing [PUT CURSOR BACK BEFORE c IN continue, BACKSPACE, SPACE, GO TO END OF ENTERED TEXT]
and add more text.

What gets saved is something like

:Enter some text
:continue writi and add more textng

More examples here https://test.wikipedia.org/wiki/User_talk:Ponor/sandbox

Adding new lines does not need to be accidental. One can add them on purpose (to make a bulleted list, for example) and change their mind, but there's no way of undoing that.