On a non-unsual Quora binge, I noticed something unusual.

After performing an operation, there was a double horizontal separator between two segments

Ok, little bug in the interface, no biggie.

But then, with horizontal separators on my mind, when I looked back at the page, I also saw all this:

Quora actually uses horizontal separators pretty heavily, but it's not immediately noticible when you glance at a page.
The thing your brain focuses on is the content. It doesn't take not of something as insignificant as a horizontal separator, but when you draw it's attention to it, you notice exactly how many times it is used.

This seems like a good manifestation of how our brain heavily filters the mass of raw data it gets from our senses, and only conciously remembers what it thinks is important.

Designers take advantage of this, and use seemingly innocuous objects like these separators to frame their content.