MyBB Community Forums

I noticed something weird in my forum. When I add stuff to forum's url, therefore creating invalid URL, my forum isn't giving some kind of "404 error" and I noticed that sometimes it can break some parts of forum (java/ajax related parts I think)

I tested disabling all plugins to see if it's caused by any plugin I use, but that didn't help.

I dont wan't user to get broken pages, any idea what's happening?

Please give some example URLs.

Basically when I add gibberish after my forum's URL, my forum isn't giving 404 error, instead I am directed to mainpage. I want my forum to give error when people write invalid URLs, I think it's messing up with stuff.

We are aware of the issue. Google SEO plugin has this option. In the future we will implement it into the core: https://community.mybb.com/thread-212936.html

Shade I think you misunderstood, I am not getting 404 error even with Google SEO.
I see that Google SEO plugin adds ErrorDocument 404 code to htaccess, but that does nothing.

It looks like whenever it should be giving 404 error, my forum redirects users to mainpage.

please paste the contents of your .htaccess for us, so that we can rule out this being an issue.