Thousands of Twitter users have received emails warning their account has been compromised by a third party.
Some accounts had been compromised, but other users had received the emails after Twitter had unintentionally reset unaffected passwords, the company said.
The mass email coincided with incidents involving several high-profile accounts, including at least one account belonging to the BBC.
Other media organisations, such as the TechCrunch blog, reported being warned.
Twitter gave no indication of the cause or source of the compromise, and would not share details of the size of the issue.
In a statement, it said: "When we believe an account may have been compromised, we reset the password and send an email letting the account owner know this has happened along with information about creating a new password. This is a routine part of our processes to protect our users.
"In this case, we unintentionally reset passwords of a larger number of accounts, beyond those that we believed to have been compromised.
"We apologise for any inconvenience or confusion this may have caused."