There are quite a handful of episodes that this show has produced that I have disliked or expressed outrage at, though to save everyone's time before this turns into a rantfest I'll just say this:
In the general sense, any particular filler episode that doesn't need to be featured. I can understand wanting to experiment with stand-alones or adding a lighthearted episode in the midst of all the angst, and sometimes they can be great, however when dealing with a heavier arc there should be less of those kinds of episodes and more focus on the seasonal arc. Which is what I have been frustrated with lately in recent seasons, them not taking things seriously. It makes the show look more like a parody of itself when they do ridiculous things. Like this season, for example, too many badfic tropes and not enough of getting to the point. Also, any episode that clearly demonstrates that the writer obviously doesn't understand the continuity of the story or the characters and throws everything off by thinking something is going to work, but it doesn't. Unfortunately, this has been happening a lot with this show lately, and it's rather disconcerting. :/
Rest of the 30 Days of Supernatural Challenge.