Another (old?) rant about open classes aka "monkey patching". There's a lot of fear and hand wringing over open classes. It's certainly a language feature that can be abused, but it's also incredibly useful. There are things you can do to be safe:

Check for the existence of methods before adding them. Don't overwrite existing standard lib methods. If you do overwrite one, alias it, call the alias from your replacement which should add something without noticeably changing the original behavior.

If you write unit tests, it shouldn't be too difficult to detect that a library you depend on has changed the behavior of a standard method in an unacceptable way. I've encountered collisions like this, maybe, a handful of times and each time finding and resolving the problem was not difficult.

I've been writing ruby software for several years now, and there are definitely libraries that annoy me (ahem activesupport), but the utility of having open classes has far outweighed any of the negatives. Let's face it, in any programming ecology there are going to be some awful libraries. Bad programmers don't need open classes to do damage.