Popular free antivirus company Malwarebytes pushed a bad update recently: here’s their corrective action statement.
“It saddens me to report that at around 3 PM PST yesterday, Malwarebytes released a definitions update that disabled thousands of computers worldwide. Within 8 minutes, the update was pulled from our servers.” (From the problem report:
http://blog.malwarebytes.org/news/2013/04/yesterdays-database-update-issue/)
http://blog.malwarebytes.org/news/2013/04/improvements-to-our-updating-process/