So! Frederator used to run an Adventure Time-related vlog that interacted with fans.
They would show production art for upcoming episodes to promo them, take fanart submissions, etc., and they would sometimes ask for fan responses to things.
So in the promo for What Was Missing, they actually acknowledged the possibility of a romantic relationship between Marceline and PB.
And asked for viewer thoughts on it.
Two unfortunate things happened. One, viewers (including me) thought that the Adventure Time team was involved with this blog, so we were overjoyed that it was being acknowledged and might have a possibility of becoming canon. There were lots of heartfelt comments and phone messages, etc. Well, the AT team was not involved with this blog, but that was not ever clarified until bad thing two.
CN temporarily pulled the episode from their website, and Frederator immediately pulled the plug on their vlog. Fred himself posted an "apologetic" message talking about how they got "caught up in all the spicy fanart," apologizing not to viewers but to AT's creators whom they prompted people to "misinterpret."
(My quotes are not exact, just scare quotes, excepting the spicy fanart thing. That was actually said.)