'Barney Quill was my father'

<< ... he stepped in the one mudpile everybody knows you have to watch out for: "never ask the witness a question to which you don't know the answer and which you are not sure how they will answer".>>

Interestingly, James Mason did the same thing in The Veridict.

1) Don't ask questions to which you THINK you know the answer...if you're wrong.

2) Do your legwork. Yay, defense team!

He did make an error, but at that point, they were desperate; they were losing the case. Their desperation is the reason they brought the lying inmate on the stand (see how the federal attorney whispers to the DA, “We have to bring him, no choice”).

Basically - He had to take a chance with that question to ruin Mary Pilant’s credibility.

It was an absolutely ridiculous and manipulative moment. He was spot on about those panties turning up out of nowhere and in reality would've focused on that. The judge would also have given the prosecution more time to respond to this sudden evidence. Why the hell would Scott start speculating about her motivations whe he hardly knows anything about the woman?

It's also misleading, because Quill taking those panties is no proof of rape, but even if it was, ultimately it's about whether Manion believed his wife was raped. At times this seemed like an episode of Matlock.