In my opinion, Rothbard's single most damaging error was a subset of / piece of particularly pernicious fallout from his "paleo strategy" fail -- his reversal on immigration.
Not because that reversal was dumb in and of itself, although it was, but because it undercut the single most valuable strategic insight Rothbard ever offered: "No particular orderism."
[L]ibertarians should push for and accept with alacrity any reduction of State power or activity on any front. Any such reduction at any time should be a welcome decrease of crime and aggression. Therefore, the libertarian’s concern should not be to use the State to embark on a measured course of destatization, but rather to hack away at any and all manifestations of statism whenever and wherever he or she can. -- For a New Liberty
What brings that to mind: Nick Gillespie's Reason interview with Dave Smith, who lays out a "particular orderist" case for not having immigration freedom until we've 1) ended the drug war and 2) wound down the welfare state.
There's actually some pretty good stuff in the interview, but the Stockholm Libertarianism really grates.