Update fails and leaves PCs unbootable

The December 2025 update was a gift that kept on giving — into January. KB5072033 would fail to install with error 0x800f0991, then leave the system in an “improper state.” Machines stuck in this limbo would later fail catastrophically when January’s update (KB5074109) tried to install, resulting in UNMOUNTABLE_BOOT_VOLUME BSODs and completely unbootable PCs. Microsoft didn’t fully resolve the chain of failures until February 2026 — two full months of systems bricking themselves because one December update couldn’t install cleanly. Happy holidays.