A U.S. federal court document asking a judge to seal documents in a criminal case unrelated to Mr Assange inadvertently revealed the charges against the Australian.
It later said: "The complaint, supporting affidavit and arrest warrant, as well as this motion and the proposed order, would need to remain sealed until Assange is arrested in connection with the charges in the criminal complaint and can therefore no longer evade or avoid arrest and extradition in this matter". "That was not the intended name for this filing", Joshua Stueve wrote.
Throughout that time, the US has refused to say whether there are any sealed charges against Assange.
Assange, who has been holed up in Ecuador's embassy in London since seeking asylum in 2012, is considered a wanted man by USA law enforcement agencies after his controversial publication of classified diplomatic cables and other secret United States government documents.
Special counsel Robert Mueller has been investigating whether Trump campaign associates had advance knowledge of Democratic emails that were published by WikiLeaks in the weeks before the 2016 election and that United States authorities have said were hacked by Russian Federation. Former Attorney General Jeff Sessions, though, had taken a more aggressive stance and vowed to crack down on all government leaks. He was detained, according to a court filing, because he "has a substantial interest in terrorist acts" that may be related to convictions against his father-in-law.
The lawyer suggested that Washington planned to impose a "grave" charge on Assange. Hours earlier, The Wall Street Journal reported that USA prosecutors are increasingly confident about indicting Assange and prosecuting him in US court.
Prosecuting someone for publishing truthful information would set a awful and unsafe precedent.
"SCOOP: US Department of Justice "accidentally" reveals existence of sealed charges (or a draft for them) against WikiLeaks' publisher Julian Assange in apparent cut-and-paste error in an unrelated case also at the Eastern District of Virginia", Wikileaks wrote on Twitter.
The filing asked the court to seal the charges, meaning they are kept secret from public view. Seitu Sulayman Kokayi was charged enticing a teenage girl to have sex with him.
The 47-year-old has always denied the allegations, and his supporters claim the accusations were an attempt to possibly extradite him to the USA, where he could face charges over WikiLeaks' publication of classified military and diplomatic documents.
He has sought refuge in the Ecuadorian embassy in London since seeking asylum in 2012.
If Assange were to leave the embassy and be arrested by British authorities, he would likely still fight extradition in the British courts.