What was Kurisu going to say to Okabe in the first episode?
Around 9 minutes into episode 24 (try watching from 08:30), Okabe from the future (episode 24 Okabe) bumped into Kurisu on the stairs in the radio hall building. He was going to say he would save her, but got interrupted by an announcement over the PA system and ran off.

In episode 1, Kurisu dragged Okabe (episode 1 Okabe) from the lecture, asking what he was going to tell her "about 15 minutes ago". He didn't know as he was episode 1 Okabe and the one she met was episode 24 Okabe.

If you don't understand how that works... Well, that's time travel for you.