I noticed that if I type "---" and press Enter to create a horizontal rule, it can never be gotten rid of. Pressing enter just creates more. The only way to get rid of them is to Select All and Clear Direct Formatting, which is problematic because I have a lot of added formatting.
Saving and re-opening the file does not help. I only tested this in an ODT format file, so it may or may not happen with DOC or other formats.
It can be undone via Undo or via paragraph border formatting. Marked as duplicate of Bug 45721, where the unintuitive nature of this feature is being discussed.
*** This bug has been marked as a duplicate of bug 45721 ***