May's update failed on machines with full disks, by design
May 2026 update failed to install on PCs with nearly full EFI partitions
The market's verdict: +2.6% on MSFT in the week after.
What actually happened
Microsoft confirmed the May 2026 security update simply failed to install on machines with under ten megabytes free on their EFI system partition, sticking at 35 percent with error 0x800f0922 and the classic "undoing changes" walk of shame. The fix was a server side Known Issue Rollback, meaning Microsoft remotely un-broke the month for affected hardware. An update that declines to update, politely, on the machines most likely to be old and full.
This one is officially acknowledged.
Were you affected? Unbreak it
Mitigated server-side via Known Issue Rollback
Microsoft has acknowledged this issue; a fix ships in an out-of-band or next cumulative update. Check Windows Release Health before manual workarounds.
Links
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Impact
- BleepingComputer (publication)
↳ caused by KB5089549
Timeline
- May 12, 2026 Mitigated Microsoft put safeguards around it.