Or rather, a big bug: "After we released Build 10581 to Windows Insiders in the Fast ring," Aul explained in his blog post, "we discovered a bug in the build that will cause the filesystem to become partially corrupted after doing a factory reset." According to Aul, this bug probably didn't affect anyone while using Build 10581, but it could cause problems when upgrading to Build 10586. Specifically, your phone could go into an infinite reboot cycle.
There is a fix: A hard reset should take you safely to Build 10586. In boldface, Aul urges users to do one important thing: back up their phones before they upgrade.
Aside from that red alert, Build 10586 offers a few smaller bugs and a long list of improvements, including:
What this means to you: This isn't the first buggy Windows 10 Mobile build. Even though Fast ring Insiders are a hardy lot, though, the big bug merits some extra care. Now that you're cautiously upgrading to Build 10586, let us know whether it was worth the hassle in the comments.