Artistic License-Statistics: Averted for the most part, but the population of Manitoba is pegged at 31.5 million in 1930, which is way more than the 4 million it sustained in our timeline at the same period.
Big Name Fan: Johnny Pez, the founder of the Sobel Wiki who generates a large portion of its content. He also regularly blogs about the book in-depth, in addition to his fan fiction.
Critical Research Failure: The conspicuous omission of the Hudson Bay Company in the description of Manitoba, and the anachronistic names for some of the Mexican states (e.g. Arizona) among other things.
Fanon: Most of the maps, the constitution and other expansions from the original text that are featured in the above site.
Fanon Discontinuity: The map that comes with the book in some editions has been subject to this in some circles.
Fridge Logic: As this post notes, the butterfly effect should have prevented Henry Gilpin's parents from meeting, considering they were 3000+ miles apart before the 1790s and met by chance in Lancaster.
It's also noted here here that Governor-General Bruce Hogg has impeachment measures drawn up against him, despite this being a parliamentary form of government wherein the vote of no confidence is clearly addressed earlier on. Similar problems exist with the surprisingly stable minority governments of John Mc Dowell and Ezra Gullivan.