While the two presidential candidates were slugging it out in the second debate many viewers thought a fly had landed on their television screens.
They were baffled because the fly landed on Hillary Clinton's face and she did not even flinch, leading some to suggest she was a robot like one of those on TV series Westworld.
Within minutes of the fly's appearance at Washington University in St Louis, Twitter was a swarm of memes, mostly poking fun at the Democratic nominee.
Many on Twitter saw similarities between Clinton and the Westworld robots.
Clinton was mocked for being robotic on Twitter. Some of the posts can be seen above.
It did not take long for someone to set up a spoof profile for the Hillary Clinton fly.
Were Ivanka and Donald Trump junior in charge of a remote control fly?
Conservative and Trump supporter designed this meme to mock Mrs Clinton.
Many people might have thought the fly was on their TV screen during the Presidential Second Debate, but it was actually on Clinton's face.
The Twitterati had endless fun with the fly, creating memes to share online.
It wasn't long before Twitter was full of wisecracks about the fly.