I should have realized this but did not - surveying is a powder keg. When the surveyor comes out and puts out his stakes and flags, it's like a siren call for neighborhood crazies to come around and dispute the line.
Basically people are retarded and unreasonable, and before just calmly talking to you, they assume you will ask them to take down their fence which is across the line (or whatever). Depending on where you live, the exact force of crazy takes different forms; out in the country you might get shot at; over in the suburbs you might get served with a notice of adverse posession. All just because you hired a surveyor, before you even consider doing anything about it. The crazies seem to see the surveyor's flag as a declaration of war.
The mistake I made was I got the survey and then I wanted some time to think about how I was going to fence things, I didn't just jam up the fences right away. That gave the crazies time, and this is what they did :
(pink flag stake is official from the surveyor of course, and crazy neighbor has put up his own line two feet into my property from the official mark)
Well done, crazy neighbor.
A bit of forehead vein popping and yelling at them got them to remove the post, but I expect more complications of this issue before it is done.
(BTW yelling at people is never satisfying in real life the way it is in fiction. In fiction there's this myth that people are actually good and reasonable, and were just doing something bad for a moment, and when you yell at them they realize their mistake and reform; like if Gorden Ramsay walked in at the right moment and yelled at Hitler he would be like "oh gosh, you're right, I'm so ashamed, I'll try to do better". Furthermore in fiction, they respond to the yelling either by yelling back, giving a satisfying argument, or by accepting the scolding and apologizing. In real life that never happens, what really happens is they try to change the subject, or turn it around and somehow blame you, or make excuses, or bring up random other points that don't matter to the issue (*), and it just leaves you feeling derailed).
(* = this is maybe the most common and most effective response - the completely random diversion into some other story that just leaves me going "WTF?" and totally takes the wind out of my sails. The other super effective tactic that I've noticed from like car salesman and contractors and such is to just completely stone wall you, like they tack on some $500 extra fee that they specifically agreed they wouldn't do, and you're like "hey WTF is this fee that you said wouldn't be there" and they just act like they did nothing wrong and of course you're going to go along with them, like "yep that's the $500 extra rapeage surcharge, so you can pay now by giving me your bank account and social security number..." , wait, what? I'm in the middle of yelling at you, you can't just act like your way is the only way).