Just tossing this out there for the heck of it.
I’ve definitely heard of people hiring an off-duty cop to make sure an ex, or other unwanted relative, didn’t show up to a wedding.
Moving this to IMHO, since this is more a question about people’s experience than anything else, and doesn’t have a factual answer.
I’m aware of a family where one sibling hired security to keep another sibling from coming to a third sibling’s funeral.
Ain’t no hatred like family hatred.
I’m not sure this qualifies as “far,” but I was once told specifically by a soon-to-be participant that I was not invited to a social event. “ASGuy, you are NOT INVITED. DO NOT SHOW UP!”
It was pretty clear that I shouldn’t show up.
So how was the party?
This cracks me up. Not only does the person think not inviting you is inadequate to keep you away, they have to say “You are not invited.” So that’s already redundant. But then they add that last line…
The only explanation for which is their belief that if you were told “ASGuy, you are NOT INVITED.", you’d take that as a dare and be determined to show up.
So, would you make a point of going to a party that you were firmly told not to attend?
Pearl Harbor?
I saw this and thought, “Moving, now that’s going to be hard to beat.” But then the response just petered out.
(Plus filler text)
In high school a friend was having a party, which he normally invited me to all his parties, but for whatever reason this party in particular was supposed to be kept secret from me despite everyone else in our friend group going. Nobody brought it up but I could obviously tell he was having a party from others who obviously weren’t told to not let me know. Got to the point one of my friends accidentally made mention of what the start time was in front of me but then acted like he was a Cold War Spy blowing his cover by having that “sudden realization” look on his face.
So the party presumably happened, I didn’t go, and the guy who didn’t invite me invited me to further parties after that so I have no idea what was so special about that one party.
I used to belong to a IRL social club which of course had little subgroups that were more friendly with each other than with the club as a whole. And of course the full club had some especially … irritating … members.
So we invented a name for our subgroup / clique: BIONICS. We’d announce to one another that we’re having a BIONICS at e.g. Fred’s Saloon at 5pm on Friday.
That is: “By Invitation Only; Never Including Carl or Stan.” Worked a charm to communicate who was, and more importantly wasn’t, to be invited. 
I used to live in a secure high-rise condo building. When a friend broke up with his certifiably insane / BPD GF he came to stay with us for a few days. I told our doorman crew that if she showed up she was not to be admitted to the building and if necessary call the cops to have her run off the property.
In completely predictable fashion she showed up, wasn’t going to take “no” from the doorman, and ended up going downtown with the police after she wouldn’t take “no” from Officer Friendly and his pals either. BPDs: there ought to be a bounty on them.
What were the circumstances behind that?
It was probably a surprise party, in your honor. With all the secretive preparations they simply forgot to invite you.
If I were that guy, I’d just do that to one person every party, so as to be extra mysterious.
I didn’t attend the event. It was a field trip for railroad historical people. I was not a historical person at the time but was interested in the depot they were going to visit. I took the “hint” and DID NOT SHOW UP.
My wife is the oldest of four siblings. Father in law is deceased and MIL is still living but is very much proud of her two “rich” children and is herself a narcissist. I can’t answer the OP’s question because we did not show up at the weddings my wife’s rich siblings did not invite her to. The sibling that did show up (he was invited but he still talks to my wife) said there was nothing to keep the “obvious uninvitees” away but then I’m sure both of the offending siblings knew nothing would come of it.
5-year-old finds out she hasn’t been invited to the royal wedding:
I always thought it was beyond bad form to show up at a wedding unless you were formally invited, unless it’s the kind of ceremony where they put up an invitation on the bulletin board at work and they guestimate how many people will show up, and it’s just a cake-and-punch reception anyway.
The wedding, or the reception? The former holds as many people as the venue is physically able to accommodate and the later has a known headcount for food, drink etc., with only limited flex.
And I’d suppose it depends on your relationship to the bride or groom or both and why you were non-invited. Psychotic ex-spouses would probably be committing a social faux pax were they to attend. An otherwise unobjectionable parent non-invited in a fit of Bridezilla/Groomzilla pique would be doing everyone involved a favor by attending and modeling grown-up behavior.
BPD, please?
Sorry.
BPD:
GF: