Activesupport is hugely widespread, yeah. It's a bit unfortunate since it provides so much, and it's rare that all (or even most) of it is used.

But beyond that though, the vast majority of issues I've seen with monkey patching has been stuff that was created in the project/company that's experiencing the problems. Libraries are generally very good about how they monkeypatch (because doing it wrong with hundreds of using-projects VERY quickly runs into problems), but those ad-hoc internal monkeypatches are routinely done by people who don't fully understand what they're doing, or take shortcuts. Those can have latent bugs linger for a very long time, and yeah - they can be nasty to unravel.