Microsoft has withdrawn a software update for its Windows Phone system after it made some handsets unusable.
The problem appears to have affected a small number of mobiles made by Samsung.
Owners reported their phones crashing, and in some cases failing to start up altogether.
Microsoft said it was working to fix the issue and would send out a new update as soon as possible.
The download in question was the first update for Windows Phone since it launched last October.
"In some circumstances it renders the phone completely unusable and can't be restored to a previous version," said Leigh Geary, editor of Coolsmartphone.com.
"It is going to portray Microsoft in a bad light," he added.
Angry users shared their experiences of installing it on user forums.
"I've got an unmodified Samsung Omnia 7, now bricked," wrote one contributor to Microsoft's Answers site.
"My phone is currently unusable, even after hard reset," another user wrote on the same site.
In a statement, Microsoft said: "We have identified a technical issue with the Windows Phone update process that impacts a small number of phones.
"We have temporarily taken down the latest software update for Samsung phones in order to correct the issue and as soon as possible will redistribute the update."
Samsung Omnia 7 owner Alex Roebuck tried to upgrade, but found his phone became unusable.
He said it was unclear if he should take up the issue with his network provider or Microsoft.
"I do not want to be without my handset, so I have decided to wait for a few days to see if a solution can be found, either by Microsoft or the hacker community," he told BBC News.
The update problem comes at a bad time for Microsoft, as it attempts to grow its share of the lucrative smartphone market.
The company recently announced a partnership with Nokia to manufacture handsets running the Windows Phone operating system.