Some bugs are caused because the programmer cannot see into the future and determine that something they programmed will be used in a manner that was not intended (using a Hammer function instead of the new Screwdriver function).
So the bug is in the code that the programmer wrote years ago. It is not suitable for today and WOULD crash and create a bug if used that way.
But is there an error somewhere? No, the programmer just cannot see into the future.
It's great that there are people here offering up opportunities to do this. There's a Rails one up there, and if you can put that on your resume, it's a plus.