tl;dr - the site will display an error if the username is wrong, but if the password is incorrect nothing happens.

When trying to log today my browser filled in the appropriate credentials, but the LOGIN button didn't have any obvious effect.

Changing the username and pressing login would tell me immediately that the username or password was incorrect, but a correct username and any password (saved & assumed correct, or a wrong one) didn't have a similar effect: pressing the login button does nothing at all.

I went through the reset process and was able to successfully log in after creating a new password.

I did a bit of testing and it seems to not do this any more, even in a private window / different browser. Perhaps it only happens with accounts that haven't logged in for a long time.

The login page is expected to display this error message in red if username or password is incorrect:

Wrong username/email or password

If nothing happens when clicking the login button it can be because the page hasn't completely loaded. Depending on network speed and Internet routing it can take 30 seconds or more before all background resources have been cached in the browser.

I believe that's how it should work, but I'm on a gigabit connection and I loaded and re-loaded and tried the page in a private window and a different browser. I checked the dev tools, looked for errors, disabled all my plugins, did all the things. For whatever reason it was not giving me an error on the screen or on the console, and it wasn't logging me in silently (Reloading the forum on a different tab still showed me as not logged in).

I'd be willing to bet it's a problem with old accounts, and that'd be a hard thing to diagnose (and probably a low priority to fix).