I love it when interviewees say "I don't know", so long as they follow up with some sort of mental process explaining how they'd find out the answer/solution. So, "You know, I'm not exactly sure how the new payments API handles excessive requests, I've only glanced at the documentation. I can look at the docs more closely and get back to you.", or even "I don't know how the new payments API handles excessive requests, but honestly, if we've reached that point, I might wanna investigate that specific issue first, and try to figure out why we're sending so many requests." - either of those responses are great, IMHO. The response I'm NOT looking for is basically, "I don't know [shoulder shrug]."