Let's take this conversation:
Alice: What is a non sequitur?
Bob: A non sequitur is something said that, because of its apparent lack of meaning relative to what preceded it, seems absurd to the point of being humorous or confusing1. In Latin, it means "does not follow".
Alice: Oh, I get it, so if I said "It is lovely weather we are having." in a discussion about the thermodynamic properties of iron, that would count as a non sequitur, right?
Bob: Yes, because cake is very tasty.
Is the last sentence a non sequitur? After all, this is a discussion about non sequiturs, it does "follow" that someone will probably make one -- which makes it not really a non sequitur.
Or is it a matter of point of view? (To Alice, it is a non sequitur, but to me, a spectator, it isn't)
1 Copied from the Wikipedia page since I'm too lazy to write my own dialogue