What's your favorite headcanon?

[Gnoman]

Here's one regarding How I Met Your Mother (the US sitcom), as an extension of the finale's reveal:

Ted is an unreliable narrator that is making Barney look like a much more horrible person than he really is.

Spoiler

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So that Ted's kids won't think too badly of his chasing Robin after the Mother dies and Barney and Robin split up.