mysterious auto format

In LibreOffice document, sometimes on a given file(I don’t know if it is file specific or not) i hit return (sometime preceded by ctrl-C or X), and a dark line is entered like a page break, i can’t select it to delete it so I use Undo and it says Undo Auto Format…I have no idea why it did an Auto Format in the first place…what is going on?

You should edit your user name: you really don’t want to use your whole email address in this way!

Are you sure you accidentally do not keep the ctrl down? (ctrl+Enter is the shortcut for page break)

That sounds like a border applied when you enter a paragraph with —, === or the like if Apply border is enabled in the autocorrect options.