An antivirus program labelled itself as malware, causing some computers it was running on to stop working.
After an update, essential components in two Panda Security antivirus programs became corrupted, leading them to be mistakenly identified as malign and quarantined.
Panda said a fix had been released and warned that rebooting affected systems could exacerbate the issue.
It asked those affected to get in touch so it could help fix their machines.
One security expert noted the irony. "People's first response is to turn [their computer] off and back on again, but in this case it seems like the wrong thing to do," said Prof Alan Woodward, of Surrey University.
The "last thing" people would expect to cause their computers to break down was its security software, he said.
Panda Security said that the signature file in both its PCOP and its Retail 2015 packages became corrupted.
"The signature file was repaired immediately. Additionally, a solution for all affected products has been automatically deployed. However, in certain environments it is possible for the incident to persist."
It added a list of steps that affected users could take to ensure that their system was clear of the issue.
Prof Woodward said the episode was "not a great advert for Panda". He said he had heard of some companies losing the use of multiple computers to the issue. He added that data loss was a risk if machines were rebooted.
But Panda Security said it was helping people whose machines had stopped working to restore them and that it had no evidence anyone had experienced permanent data loss as a result of the issue.
A Panda Security spokeswoman said: "We have solved 90% of the incidents, and support is being given in real time to the ones that still have some issues."
The company said that only 8% of the "millions" of PCOP and Retail 2015 customers were affected by the issue. A spokeswoman refused to be more precise about the figures.