2000 and Bush Gore. They had three official recounts. The number was always different, but not by much .. but it was still higher than a 1% margin of error. The FL ballot was pretty garbage and since then, most states have avoided punch-out/butterfly ballots. The Supreme Court ended up appointing Bush and the nation's voting commissions learned an important lesson.
But like OP said, volunteers could see the process. The recounts were actually pretty damn close; it was just the election was way too close for that to be acceptable.
In general the US system, as crap as it is (first past the post is garbage; no possibility for 3rd parties as with ranked or MMP systems), the electoral and state-based rules do mean that election fraud isn't too big a deal. New York will likely always go Blue. Tennessee will always go Red. Even if there is same fraud there, it doesn't matter that much because of the way votes are allocated per state. It's not fair, but that's the system the US has.
Fraud matters greatly in states were elections are close (Florida, Ohio, Arizona, etc.) and it will get even worse thanks to National Popular Vote Interstate Compact, if that gets triggered.
Potential fraud in swing states or NaPoVolIterCo are incredibly dangerous right now, because what we cannot have in America is loss of faith in the election system. That will be disastrous. We would not survive the 2000 Bush/Gore election today.