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I just came back from a week vacation, and I found many deep pits in my cactus! They're small, but not shallow, and in each of them, there's red! Obviously, my first thought is Red Spider Mites, but a similar thing happened to this same cactus a while ago, and it was just sunburn. I've got to cure this cactus - please tell me what it is! Worth noting also is that ALL of these spots occur at the same "latitude" of the cactus. Like, all the spots would line up on a horizontal circle drawn around it. It's above the "equator" of the plant, and they're all BETWEEN ribs, not on tubercules. Total, there are six of these pits.

Strange. There might be some kind of rot or bacterial or fungal infection in that spot. Is it soft? If that's the case, you could possibly try cutting below that spot and letting the top regrow.

Very unusual. Im pretty sure its not mites, mealies or other parasitic bugs, it looks very mechanical and there seems to be a bit of damage like it around the areoles onn the left of the pict too. I think there may be some larger herbivourus bug attempting to eat it but then the red colour seem weird. It could be some radom oxidation reaction to the air like when you leave a half eaten apple out and it turns brown. I would be tempeted to cut the top off just below the lowest red mark. It could be rotton inside and only now the outside is catching up.

Just had another though, where do you keep it? is it in a window. could be reflection burns with the straight line caused by the movement of the sun during the day.

Yes, yes, I do keep it in a window! And the three most major marks were on the side of the plant facing the window! So it doesn't look like it's characteristic of any pest? I do know that Echinocactus grusonii is very prone to sun damage, so I hope it's just that. I've been keeping it out of the sun recently, and it doesn't look like the spots have grown. And Adetheproducer, the other spots on the tubercules ARE just sunburn, but thanks for your concern! I'll update you again in a few days. Thanks a lot for your opinions.

Orange (rusty) spots could indicate a fungal infection, I'm having a big problem with my plants it's been more than a month, it's a fungal infection (Fusariosis caused by Fusarium oxysporum), the first signal of infection are the orange spots, but these appear mostly very close to the areoles.

In you case they look a bit different, but could be something similar.
It doesn't look like sunburn to me, they usually show signs of sunburn on the most outer (exposed) parts of the plants.

Also, E.grusonii should be very resistant to sunburn, even some young plants I have here are exposed to harsh full sun all day, they never showed any kind of sign of sunburn similar to that, when there is too much sun, they usually just show a more pale green.

I hope you solve the problem.