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Rake::RemoteTask (a dependency of Vlad) defines a bunch of stuff on Kernel, including #role. For whatever reason, if an ActiveRecord model includes role as a property, things break in weird ways.

Reasonably easy to work around by aliasing/undefing Kernel#role to #server_role or something, but amusingly that would be something that could be fixed by refinements (though I'd rather see something more like Thor, that's not run toplevel).

I ran into a nasty bug a while back which came down to a conflict between two JSON libraries in their definitions of #to_json on core classes.

I have seen issues quite a few times, and they are serious headaches to fix when they come about. I've had libraries implement Rails utility methods slightly differently, or conflicting implementations of monkeypatches on S3 libraries.

While it's not incredibly common, disrespecting namespace concerns is a sure road to problems.