Minato tells Kakashi in Kakashi gaiden that the chidori is dangerous because his eyes can't follow his opponent's counter attack. It's assumed that Kakashi has completed the chidori after he gains the sharingan, however I think it's more likely it took him much longer. I don't think Kakashi intended to hit Rin. I don't think Rin is a spy or that they're commiting suicide to protect sensitive information about Konoha. I think Kakashi has grown careless and arrogant since he obtained the sharingan. I think there is a substitution jutsu going on here or Kakashi is moving too fast for his own good again. I vaguely remember Kakashi saying something similar about the dangers of chidori and it being a jutsu best used for assassination when he was teaching it to Sasuke. Basically I'm saying this should have been obvious to us all when there has been so much foreshadowing about the dangers of chidori, while there haven't been any real examples of why it could be a double edged sword. We really should have realized Rin was killed via chidori long ago.
Someone else brought this up recently too:
As for hunting down the actual panels... Look that stuff up on your own. It's not like there are very many chapters featuring both Minato and Kakashi... It should have been obvious where to look. If you can't participate in the conversation here without providing citations from the manga and dissertation length essays on the Narutoverse, I'll go back to lurking. Ever heard of Foogling? No one here is your secretary...