There is a user who seems to be (systematically?) asking a series of basic questions and then, apparently as soon as someone answers them, deleting them before they receive an upvote (or, obviously, accepting the answer).

I only noticed this because I voted to close one of their questions after it had received an answer (classic typo: [$arg) and then they reasked the question in a slightly modified form, armed with the information provided by the previous answer. I tried to mark it as a duplicate only to find the original was gone...

This strikes me as the worst type of help vampirism: poster gets what they need and then leaves nothing for the community.