I have this problem when playing single player. I think it's a plain bug, but I have no idea what causes it.

The camera looks straight up when I click the mouse to do mouse look. It doesn't rotate upward, it is just suddenly straight up. Whether there is something else that sets up this situation, I don't know, but I'm sure it can happen when I click and I'm not pressing any other keys at all.

I'm sure it can happen without mods. I don't know if some mod could cause it to become more frequent.

It can happen without keyclone.

It can happen in fullscreen or windowed mode.