This isn't really a game ruining bug but it is a bug nonetheless. Was checking out the Shipyard Sewers and encountered this:

http://steamcommunity.com/id/novoro/screenshot/560922580895428305?tab=public
http://steamcommunity.com/id/novoro/screenshot/560922580895426220/?tab=public

Some mutant was stuck behind the wall, unable to roll out of the pipe. As shown, he resorted to tossing knives at me through the wall.
Nothing to cry about, but still should be noted.

I had the same issue.

I've come across this too. Very random chance of it happening in most of the sewers.

Happened to me to in the exact same place. Confused the hell out of me.

Yeah, happened to me in Kavasir's sewers... Really freaked me out, thought i was facing an invisible mutant... But it did not count towards cleaning out the sewers at least.

Peace and Love!

Happens all the time in Shipyard Sewers. Likely easily fixed in the next patch if a dev took a sec to read, acknowledge it and fire up his game editor.