system
A chat on subreddit /r/NoStupidQuestions
human
&gt; i once heard a story about a ram whose horn curved into its own brain

That's not really an issue of evolution though.  That's an individual animal where something went wrong.  Evolution doesn't happen in individuals, but populations.
gpt
Well, it kind of is. That type of ram (and many, many, many other animals) have horns or tusks which have evolved to continually grow forever. Generally that's a good thing, but if the horn doesn't get worn down or broken off it can grow into the animal. 

Not an evolutionary "failure" per se, but an effect of evolution backfiring