category permalinks not always working

Try resetting your permalinks by first setting to default and then to your current settings.

No luck.

Can you deactivate all plugins, revert to Twenty Twelve theme and try again?

BTW, are you sure you updated to WordPress 3.5.2 ? Can you link to your site?

3.5.2 confirmed, 2012 theme, no plugins active: https://farzher.com/?cat=42
Pretty weird.

I find it’s OK. cat=42 is tutorial (https://farzher.com/category/tutorial/)
Now, what do you want to change or what kind of mistake do you find (apart from some edited code showing up)?

You’re saying you can’t visit https://farzher.com/?cat=42 because it redirects to https://farzher.com/category/tutorial/ ?

That’s all I want it to do, is force a redirect, like ?cat=43 does. When I visit ?cat=42 it doesn’t force a redirect.

The first link is the category ID and it resolves to its slug tutorial. Both ways it’s OK as long as the same page loads. It becomes a problem only when the assigned page does not load or if you get a 404 error.

No, it’s not okay; it’s bad for SEO. Two indexed pages with the same exact content. I don’t want ?cat=42 to exist.

Either way, why is it doing that? Is it supposed to? I don’t get it.

No, it’s not okay; it’s bad for SEO. Two indexed pages with the same exact content

No. It’s the same page, not two pages. Click on your links or create a sitemap and see how many pages you get. You will get only one.

I don’t want ?cat=42 to exist.

Delete it if you can. It’s your site. But you will lose the second one also.

Either way, why is it doing that?

You will get a lot of stuff explaining that here.