SAP referred to the TomorrowNow customer subpoenas in one of a series of recent filings related to discovery in the sprawling suit.

"It's a key point to any resolution whether Oracle's bottom line was damaged by former customers turning to TomorrowNow for their [application] support needs," 451 Group analyst China Martens said via e-mail Thursday. "Oracle seems to be struggling to prove that point, hence the resorting to customers. We question the wisdom of that move, which seems overly intrusive to end-users."

But one attorney who has been following the case said Oracle's move is not unheard of, and was "very appropriate" given the circumstances.

"So much of [SAP's] defense is that [TomorrowNow was] acting on behalf of their customers, so naturally Oracle wants to know more about the customer interactions," said Eric Goldman, an associate professor of law at California's Santa Clara University School of Law, via e-mail.

That said, sending a defendant's customers subpoenas is "usually a hard-ball tactic, because it annoys customers a lot (it means they have to spend time and money to deal with someone else's lawsuit) and can spook customers from dealing with the vendor," he added.