It would be good to include the caller in this so that users can see where it's coming from.  Also I tend to put "WARNING" in my messages like this so it's clear what kind of message it is.  Maybe something like this?

``` ruby
Kernel.warn "WARNING: ignoring assignment of `pattern` option to #{value} since RSpec has already loaded the spec files. Called from #{caller.first}"
```

(You should verify that `caller.first` is correct -- I think it is since we don't call this  internally anywhere.)