[vundercind]

My specific example is just Bruen with the serial numbers filed off.

Factual errors are downright common (a Google will turn up many efforts at fact-checking the "facts" in Supreme Court opinions) and aren't a new problem, dating back basically forever.

One key problem is that they simply don't have the resources to check all their facts. Their sources of facts are often amicus briefs from interest groups and specialized court-lobbyists, drawn from large piles and quickly skimmed for relevance by overworked early-20s law clerks. It would be surprising if they didn't get things wrong all the time. They do—luckily, a lot of times, it barely matters, but sometimes they get facts wrong that were central to their opinion.

The point is, don't believe a "fact" you read in a court opinion without double-checking. Even if it's about legal history.

[EDIT] As my sibling commenter points out, it's also the case that sometimes they just lie on purpose. But even absent that, the circumstances under which opinions are written would generate factual errors by accident, with some frequency.