"Online" is the alt text you see when the little image to the left of someone's username doesn't load.
I'm talking about these images:
In the HTML they look like this:
<img src="https://www.eevblog.com/forum/Themes/default/images/useron.gif" alt="Online" data-pagespeed-url-hash="159412188" onload="pagespeed.CriticalImages.checkImageForCriticality(this);">
You can manually trigger this "Online" bug by editing the page in your browser (right click page -> Inspect or Edit) and intentionally adding a typo into the image URL, so it can't load.
Possible causes:
(1) URL to image is wrong (unlikely? but possible if eg "theme" is randomly changing)
(2) Image is not being received by the web browser when requested. Possibly a forum (PHP?) bug, webserver issue, caching issue, browser issue, your own internet link, could be anything.