XKCD Isn't Funny - #1677 - Contrails
So apparently 'contrails' is a super commonly known term and I just had no idea. Welp, there go my grounds to criticize this for using obscure words.
There's a similar joke that happens in the Friends episode "The One With The Fake Monica" where Chandler convinces Joey to use "Joseph Stalin" as a stage name. With this, we can easily visualize the consequences of the misdirection. Joey will not get the parts he's looking for because he's using the name of a terrible dictator.
In this comic, things are left a bit more vague. Someone could mistake him for a conspiracy theorist, but that's only if the topic of contrails comes up in conversation, which strikes me as pretty unlikely. Maybe I'm just a sadistic guy, but I don't think lying to someone is funny if nothing happens because of it.
Also, wouldn't it be obvious that a person is from the UK based on their accent?