It started when Cook was asked what he would do if he were faced with the problems facing Facebook after the Cambridge Analytica scandal.
"I wouldn't be in this situation," Cook said in an interview with Recode's Kara Swisher and MSNBC's Chris Hayes last month. The Apple CEO questioned the practise of monetizing user data on social media platforms.
Zuckerberg then hit back days later, saying that the service should be free.
"You know, I find that argument, that if you're not paying that somehow we can't care about you, to be extremely glib. And not at all aligned with the truth," Zuckerberg said in an "Ezra Klein Show" podcast published Monday by Vox.
There hasn't been much else said since then, but Zuckerberg was ready to take on Apple if lawmakers asked him about it during his testimony on Tuesday in front of the Senate Judiciary and Commerce committees.
That is according to his notes, which were photographed by Associated Press photojournalist Andy Harnik and posted online by a CBS editor, Stefan Becket.
One section of the notes was entitled "Tim Cook on big model."
But then the notes prepared the Facebook CEO to attack Apple's track record with apps.
"Lots of stories about apps misusing Apple data, never seen Apple notify people," the notes said without elaborating.
Apple was not eventually mentioned in the testimony.
An Apple spokesperson was not immediately available for comment.
Below is a full copy of the notes.