Symbolic Validations

When exactly don't you have to :"escape" a Ruby symbol?

Because this question is somehow related to the Ruby interpreter's internal usage of symbols, the rules are not the most obvious ones:

¹ Valid for identifiers: A-Z, a-z, 0-9, _, non-ASCII characters. Not allowed to start with 0-9.
² Example of a syntactical edge case that is not 100% clear

More Idiosyncratic Ruby