Just came across this funny description about runaway code:

http://www.nczonline.net/blog/2015/05/14/the-bunny-theory-of-code/

I'm guilty in enabling such bad habits at times (copying without questioning), who else? Just be careful it doesn't turn into a killer rabbit...

While I haven't personally hit that case (I'm neurotic about my own check-ins and thoroughly review anything I take from elsewhere), I have seen it with libraries. We had a utility library that contained a bug in XML serialization that was intermittent and not caught for over a year. By the time I found it, the library was used in a number of applications that then needed to be rebuilt and reversioned, with a not insignificant amount of effort for us and our customers. Bad times. :(