During her testimony last week, Heard referred to Kate Moss.
Insider was told that it could open the door to fresh evidence on past relationships.
Heard was arrested on a domestic violence charge in 2009, but her ex-girlfriend said it was overblown.
Before she left the witness stand for a week, Heard talked about the fight she had with Johnny Depp after she discovered text messages that showed he was unfaithful.
She mentioned a person who was not expected to come up in the trial.
Heard said she screamed at Depp during the fight because he was punching at her sister, who was standing on top of a staircase.
I don't hesitate. I do not wait. Heard said that he swung at him because he thought of Kate Moss.
Heard was referring to a rumor that said that Moss was pushed down the stairs by Depp when they were dating in the 1990s. In later interviews, Moss has spoken fondly of him.
According to documents reviewed by Insider, Moss isn't on the list of witnesses, but he did reserve the right to call any witnesses needed for rebuttal or impeachment.
Lawyers usually work out the evidence before the trial begins. Lawyers on both sides spend months searching the other parties' files and interviewing witnesses in order to find evidence that can be used in the case.
Heard and Depp are facing off in defamation lawsuits over whether one abused the other during their relationship and marriage. A source close to the case whose identity is known to Insider, but who requested anonymity so they won't affect the ongoing trial, said that the motions establishing the precise scope of evidence were filed under seal.
Heard mentioned Moss and the alleged staircase incident, as well as describing injuries she may have suffered from a fight and her own guitar-playing skills.
Benjamin Chew was excited when Heard mentioned Moss.
Heard is expected to finish testifying on Monday, and the mention of Moss is likely to come up in the cross-examination, according to a source familiar with the thinking of Johnny Depp. Heard only made a passing reference to Moss, so it may not reach the standard of relevance just yet to have the model testify.
This person said that if Heard said she thought about Moss, the lawyers for Depp could ask the judge to call Moss to the stand and end the rumor.
If this is a rumor and Johnny did something to Kate Moss, the jury is going to wonder if he did anything.
According to a person familiar with the thinking of Johnny Depp, his lawyers would try to get the judge to allow questions about Heard's ex-girlfriend Tasya van Ree.
Heard was accused of hitting Van Ree in the arm during an argument and was arrested at the Seattle airport on a domestic violence charge.
The charges were later dropped and van Ree released a statement. Heard was wrongly accused for an incident that was misinterpreted and over-simplified by two individuals in a powerful position.
Christa Ramey, co- founder of Los Angeles-based civil litigation firm Ramey Law PC, told Insider that Heard's mention of Moss may have gone against her legal team's wishes.
Sometimes I have clients that think they are smarter than me and they go rogue on the stand. Ramey said she definitely went that route.
Heard may have wanted the jury to know the reference. Ramey said that it happens when jurors are not allowed to consume news about trials.
Every time I start a trial, my searches on the internet go up. Ramey said that jurors are not supposed to search for stuff all the time.
Heard did it anyway, even though the judge didn't want rumors or previous relationships to enter the case. The judge has to consider what other information may be brought in the case out of fairness to the other side.
If an attorney or a witness decides they want to just run the risk of going against my ruling and hoping that everything will stay the same, it puts me in a very difficult position of needing to maintain the integrity and fairness of the trial.
The wording of Heard's name drop was tooskillful to have happened without the help of her legal team, according to the co-chair of the matrimonial and family law practice at Blank Rome.
He said that Heard was able to avoid the legal team objecting to the testimony because she said that she thought of Moss in that moment.
Ward said that it may turn out to be a mistake since it opens the possibility of the legal team calling Moss and other ex-girlfriends to give glowing reviews of his behavior on the stand.
Moss isn't sure if she's praising Depp or not. She has not publicly addressed the staircase incident. In a 1998 interview with Rolling Stone, Depp said that he told the writer his own romance with Moss.
He zeroes in on faults and good points immediately, and I think he went straight for the romance jugular. I think I told him that she gets a severe beating.
Other people have said that Depp was a good boyfriend.
The ex-girlfriends of Johnny Depp wrote statements of support for the actor during his libel lawsuit against the UK tabloid The Sun, which had labeled him a wife beater. The judge ruled that Heard's allegations were substantially true.
She said that she had always known him to be a non-violent person and that she had dated him for 14 years.
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She wrote that she was absolutely shocked, confused and upset to hear about Heard's allegations, because she was never violent or abusive during their relationship.
I don't want to call anyone a liar, but from my experience of Johnny, it is impossible to believe that such horrible allegations are true.
Usually character references like this in a trial are off-limits, because the court recognizes that just because Depp may have treated an ex-girlfriend well, doesn't mean he didn't abuse one of them.
Heard may have made similar mistakes elsewhere in the case, such as when discussing the aftermath of an explosion between her and Depp.
The two stayed in a house in Australia as they filmed the fifth movie in the Pirates of the Caribbean series. The night of March 7, Heard threw a glass bottle at him, which shattered on his hand and sliced off the tip of his middle finger. Heard testified that she was sexually assaulted with one of the bottles, and that he cut off the tip of his finger while high on drugs.
One of the photos entered into evidence showed a guitar with blood on it. He testified that he wouldn't cut his own finger off because he loves the guitar so much. Heard was asked if she was responsible for the streaks.
Many of the fans of Johnny Depp pointed out that Heard played the guitar. She plays an aspiring rock star in the movie One More Time.
The judge would have to decide if the fact that she's acting in the film meets the appropriate standards of evidence if the movie trailer or clips were to be entered into evidence.
Heard said that during the Australia fight, her feet were cut off by the broken glass on the floor from the bottles and windows that she said Depp smashed.
Heard didn't seek medical care and didn't appear to have injuries, according to witnesses who were called to the stand. Heard will have to face that discrepancy in his cross-examination.
The person close to the actor told Insider that the guitar and feet issues were an embarrassment of riches, and that the actor's lawyers may bring them up in cross-examination.
Heard's attorneys pointed out that none of the people involved in the incident were focused on whether Heard was injured. Their jobs were to protect and care for him.
The judge will have to decide how much outside evidence she will allow into the case.
If a key witness in this case says something that impeaches something a witness has said, most judges would allow at least some conflicting evidence to be admitted.