'break' in method context should emit a parse SyntaxError. We will parse it and then throw a LocalJumpError if we ever run into it.

def foo
  if false
    break
  end
end

foo

The severity is very minor since most people will not write code like this unless they are JRuby-only, but it should not be difficult to track down since it happens during parse time at some point (we just need to find the error message in MRIs source).