YouTubers or internet personalities saying something ill advised is nothing new. It just somewhat happens since these people came out of their bedrooms and not through PR or media trained avenues most of the time.
Felix "Pewdiepie" Kjellberg has found himself the center of attention though after making a joke about being part of ISIS. It's a bit of a strange story about how it got to it, but essentially, Pewdiepie decided to unverify himself on Twitter, a fake news story started circulating that he was unverified by Twitter because he had ties to ISIS, he joked about it, people got mad. This caused Twitter to temprarily suspend Pewdiepie's account.
You can see his take on what happened here:
Obviously, it's a dumb joke, and one that is problematic for a figure so many young figures look too. On the other hand, it was clearly a contextual joke about what had been happening to him after he unverified himself. Still, that context obviously gets lost if you just see that tweet, and with millions of followers, that is bound to be done a lot. What was clearly a joke between friends gets seen by many, is rightfully pointed to as ill thought out, and we get to where we are.