I've always hated the term "special characters." If you are a non-technical user, it leaves a lot to the imagination.
1. What characters count as special? Does "special" mean non-alphanumeric? If so, just say that, don't say "special".
2. Among those, which ones will be accepted by your password system? Is \ acceptable? Will ™ be accepted? What about emoji (which evidently HN comment input doesn't seem to accept)?
Most password entry prompts do not answer either of these questions, so you have to either guess or just use ! or # and move on with your life.