I'd mostly be worried about that case if they, afterwards, decided to sue me for copyright infringement of my own code. I expect that that could be a difficult

I'd mostly be worried about that case if they, afterwards, decided to sue me for copyright infringement of my own code. I expect that that could be a difficult situation to explain in court. "Yes, your honor, I wrote the software, gave it away for free, and said that someone else could take credit." It just sounds weird, and I think it'd be a somewhat foreign concept for most people. If I'm putting my stuff out on WTFPL, it basically implies that I'm already OK with someone taking it, relicensing it, and distributing it under a new license (and even under a new attribution). Personally, I would have a problem with it, so I've never licensed anything under WTFPL.