http://imgur.com/a/W8NxE

I added RSO to an existing save long ago and I just noticed this. I am on the latest version now.

Thats.. unexpected :D
Is it possible for you to share the save and all the mods for it?

It looks like rng seed fails to generate properly for given coordinates. HAven't seen it happen before so can't do much without mods and save.
Also can you try to call regenerate command (details are on forums in mod info) and see if it will recreate this ore patches or make new proper ones.

Generating a new seed and regenerating resources worked, without generating a new seed made it even worse, as those patches from the screenshot spawned everywhere.
But now I gotta deal with all those biters inside my walls. :D

I probably should mention that I started this save back in 0.9.8 and kept upgrading and adding more and more mods.

Ohh thats an ancient save then :D
Resource gen in RSO changed quite a few times over time so a lot strange things might have happen.
It's quite possible I did not check for something from older versions.
Let me know if it repeats again.

It is, but I added RSO not that long ago, I think 3 or 4 versions ago.