example: https://www.gog.com/forum/general/thirdparty_gog_library_downloaders/post3

I've only seen this in my own messages, but I'm not active on the forums. May be related to using links in messages.

Can't tell you exactly what causes it, but I think it has to do with the last word on a line not fitting in. With that message in particular you can fix it by editing the message, scrolling the cursor to the first ' [ ' of the link and pressing ENTER. That will force the link to appear on a new line and in the process sort out the weird spacing of the line preceding it.

That's caused by what the forum considers (overly long) a single word without spaces. In the case you highlight it's because the url is (lazily) both link and description (spaces added because of dumb forum formatting):

[ u r l = https : // etc...]https : // etc...[/ u r l]

Instead of

[ u r l = https : // etc...]Description.[/ u r l]

Had it been done with a brief description it would have come out like this:

" I've found an informative thread: Informative thread "

Or even better:

" I've found an informative thread. "

Not even the forum, but simply your browser. Forum posts are formatted with text-align: justify; which causes all but the last line in a paragraph to take the full width of the post, with spaces between words lengthened proportionally to make that work.
A long string like a URL is split by slashes, but not by underscores, so everything after /general/ in the URL is treated as a single word. That's too long to fit on the first line, so goes to the second line, and what's left on the first line is then justified, resulting in those huge spaces.

Agreed on what would be a clean solution for that post.