"I'm very sorry right off the top of my head, I know that there are a lot but, uh, I'll put it up on my website I promise you," she said.
Prompted by CNN's Wolf Blitzer for a better answer, O'Donnell spun and talked about Roe vs. Wade, pornography and then about federal court decisions regarding Don't Ask, Don't Tell and Mirandizing terror suspects.
A moment later Coons pointed out that they'd already discussed the recent Citizens United campaign finance decision during the debate. (He disagrees with that one.)
(Update: O'Donnell spokesman Dave Yonkman told reporters after the debate that "She was caught off guard." He said O'Donnell disagrees with the high court's 2005 Kelo vs City of New London decision expanding eminent domain authority.)
As TPM posted earlier, O'Donnell also confused her history about the United States' role in Afghanistan.
And, in another pointed exchange, O'Donnell struggled to explain her comments in 1998 that she believes "evolution is a myth." She said that, in that clip aired on "Politically Incorrect," she was talking about what a local school taught. She declined repeatedly to answer moderators' questions about whether she believes evolution is a myth.
"What I believe isn't relevant," O'Donnell said. "What I will support in Washington D.C. is the ability for the local school system to decide what is taught in their classroom." She added that she thinks it is against the Constitution and is "overreaching" for the federal government to block schools from teaching creationism in tandem with evolution.
O'Donnell rolled out what seemed to be a scripted laugh line early in the debate, telling Coons: "You're just jealous that you weren't on 'Saturday Night Live.'" (See the SNL skits here.) She also said that she has "not welcomed this media attention," and noted that Blitzer had been chasing her for an interview.
Coons said O'Donnell wants to take Delaware back to the past and that her "extreme positions" aren't in line with their state.
Coons also said that he doesn't think the Bush tax cuts should expire in full, and that he does not agree with Obama and most of the Democrats that $250,000 should be the dividing line for the wealthy and middle class.
Watch O'Donnell ask for a hint on the Supreme Court question:
Coons remains a strong favorite to win this race, with the TPM Poll Average showing him leading O'Donnell 56.7%-37.4%.
But Obama and Vice President Joe Biden are campaigning for Coons Friday, just in case.
Additional reporting by Rachel Slajda