My internet connection kept dropping out and coming back, but unfortunately I was fairly committed to upgrading with half the files already uploaded via FTP. I did manage to complete the FTP transfer after 3 tries, and then I began the guided upgrade, and as templates were being rebuilt, again my internet dropped out. I nearly tore my hair out as I waited for the internet connection to come back.
When the internet did come back, and I hit refresh, I was delighted to see an "Upgrade Complete" message, and my file health check was error free. Everything seems to be working just fine now. Thanks XenForo for making such a rock-solid product.
Now, if only my internet service provider was so reliable!