You know the thing that has always fascinated me about words like this is when they, illogically, only have a two word function. I mean, have you ever heard someone talk about a sequitur? I haven't, I've only heard it used with the non in front.
A non sequitur is a statement that does not follow logically from or isn't clearly related to previous content.
This post was a non sequitur from the previous ones about phobias.