For what it’s worth, I ran into the unwanted-interstitial risk. At one stage, before I’d bothered registering jmason.org, sitescooper.taint.org or my other domains, I used a URL-shortening service to provide a memorable, short URL for an open-source application I wrote — http://zap.to/snarfnews/.
At some point a few years down the line, the forwarding process started accreting ads; eventually they became soft-porn in content, and I was forced to apologise to users for the forwarding I could no longer control!
By now, 10 years down the line, it seems to hijack the page entirely, returning a page in Cyrillic I can’t even read :( (apparently it’s a page of Flash games; thanks, Alexandr Ciornii, for the interpretation!)
Anyway, lesson learned.