Twitter CEO Jack Dorsey apologized to the followers of former U.S. President Barack Obama on Saturday, after a glitch forced some users to follow Donald Trump’s new Twitter account.

Dorsey said that technical issues were behind the problem, which outraged many users who accused Twitter of working in favor of the new Trump administration.

In a series of tweets, Dorsey explained the technical reasons behind the glitch and said that over half a million Twitter users were affected.

Prior to Trump’s inauguration, the White House had planned to transfer Barack Obama’s @POTUS account to @POTUS44, with its followers in tact, but Twitter mishandled the transition and followers of the @POTUS account automatically followed both accounts.

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