I disagree with this. At least from my own perspective, numbered list imply one thing: that I can expect a specific number of items. In fact, in the given exa

I disagree with this. At least from my own perspective, numbered list imply one thing: that I can expect a specific number of items. In fact, in the given example, the non-numbered version of the title sounds like it should be more authoritative. It goes from an interpretation of, "a list of 20 common mistakes," to one of, "a list of /the/ common mistakes." I.e., to me at least, the second option has an implication of being a list of all the common mistakes, while the former implies that this while this is a list of 20 mistakes, there could be more. It is the adjective ("amazing," "killer," "must-have," etc.) present in the title and NOT the number that conveys authority, completeness, etc. Perhaps I am the exception, though, and not the rule.