Not a flat you live inside, like in the UK – but a flat piece of cardboard.
Source Appleshadow
Remixed from a drawing in 'Line and form', Walter Crane, 1914.
Source Firkin
Looks like a technical drawing board: small squares forming a nice grid.
Source We Are Pixel8
Heavily remixed from a drawing in 'Barbara Leybourne; a story of eighty years ago', Sarah Hamer, 1889.
Source Firkin
A repeating background with dark brown stone-like texture and abstract pattern that looks like tree trunks.
Source V. Hartikainen
8 by 8 pixels, and just what the title says.
Source pixilated
A textured blue background pattern with vertical stripes.
Source V. Hartikainen
It almost looks a bit blurry, but then again, so are fishes.
Source Petr Šulc
The following repeating website background is colored in a blue gray color and resembles a concrete wall or something similar to it.
Source V. Hartikainen
Black & white version of a pattern that came out of playing with the 'light rays' plug-in for Paint.net
Source Firkin
A smooth mid-tone gray, or low contrast if you will, linen pattern.
Source Jordan Pittman
And some more testing, this time with Seamless Studio. It’s Robots FFS!
Source Seamless Studio
A large pattern with funky shapes and form. An original. Sort of origami-ish.
Source Luuk van Baars
You know you love wood patterns, so here’s one more.
Source Richard Tabor
Sort of reminds me of those old house wallpapers.
Source Tish
Tiny little fibers making a soft and sweet look.
Source Badhon Ebrahim
You don’t see many mid-tone patterns here, but this one is nice.
Source Joel Klein
Prismatic Groovy Concentric Background 2 No Black
Source GDJ
Prismatic Curved Diamond Pattern 7 No Background
Source GDJ
A seamless texture traced from an image on opengameart.org shared by Scouser.
Source Firkin
From a tile that can be had by selecting the rectangle in Inkscape and using shift+alt+i.
Source Firkin
Remixed from a PNG that was uploaded to Pixabay by VictorianLady
Source Firkin
Very dark pattern with some noise and 45-degree lines.
Source Stefan Aleksić