In fact, Google's lawyers never should have produced the email in the first place. Later that evening they filed a motion to "claw it back" on the grounds it was "unintentionally produced privileged material." Oracle objected, and so began a three-month battle that culminated last week with Alsup's refusal to exclude the document at trial.

Lindholm's computer saved nine drafts of the email while he was writing it, Google explained in court filings. Only to the last draft did he add the words "Attorney Work Product," and only on the version that was sent did he fill out the "to" field, with the names of Rubin and Google in-house attorney Ben Lee.

Because Lee's name and the words "attorney work product" weren't on the earlier drafts, they weren't picked up by the e-discovery software as privileged documents, and they were sent off to Oracle's lawyers.

If the drafts hadn't been skipped over, it's entirely possible Lindholm's email never would have come to light, legal experts said. In the discovery phase, each side draws up a "privilege log" that lists the documents they're withholding, with a brief description of why they're privileged.

It's not rare for attorneys to challenge items in the privilege log, but it's also not an everyday occurrence. Since the Lindholm email was addressed to a Google attorney, it's entirely possible Oracle never would have challenged it, meaning its content never would have been made public, said Stephen Hall, a partner with the law firm Bradley Arant Boult Cummings.