So, I haven't read any Cass stuff (I will, I promise!) but with my tenuous understanding of her origin story in mind, I wanted to take a closer look at this exchange from a Death in the Family.
Jason has reason to believe that Shiva Woosan could be his biological mother. Batman helps him track her down, and interrogate her:
Shiva didn't know what this was all about, and to be fair, that was an unexpected direction for this line of questioning to go in, but I'd like to posit something here: Look at her face. Look at her eyes. That's a lot of expression from someone as well trained and thoroughly guarded as Lady Shiva.
I wouldn't expect the world's deadliest assassin to react that strongly unless.... she has had a baby. One that no one was supposed to know about. One that she's been trying to forget for 15 years. How does she cover up this little slip? Deflect with some good old fashioned sarcasm:
Which prompts Batman to do something drastic:
"That's the biggest trouble with this line of work--" he tells us on the next page. "You can't always get the job done, and remain a hero in your own eyes." That line resonates so strikingly with Jason's story later on, but this isn't about Jason.... it's about Cass... maybe.
Batman asks her again, "Have you ever had any children?"
So if we're going by comic book logic here, this truth serum is 100% effective, and Lady Shiva is not Jason's (or anyone else's) mother. The trail has run cold and it's onto the next one.
On the other hand, if we take a more realistic approach, we could say that Sodium Pentothal is a fast acting barbiturate that weakens the subject's resolve and decreases inhibitions. Under it's effect, Shiva would be more compliant, and who knows? She might even tell the truth; however, this drug is not just a truth serum.
By perusing the wikipedia page, you can see that this drug has also been used by psychiatrists to desensitize patients and to "facilitate the recall of painful repressed memories." Memories like leaving your baby to be raised by a madman as a weapon.
What if those weren't just incoherent groans in that second panel? Maybe it was closer to something like "...Cugh... eh...neh...". What if, before regaining control, she started to say "Cain"?
I don't think Shiva was saying "no, I've never had any children." I think she was saying "no, I don't want to remember."
So I fixed it. Cassandra Cain existed in 1988, no retcon necessary. (Did this scene get addressed at all when we discovered who Cass’ mom was? Let me know!)
From BATGIRL #65. They don’t address the truth serum part or why it didn’t work, but they sure flashed back to it.
Wow, this is EXACTLY what I was looking for. You are amazing!