I have a nasty little habit during sermons. While I'm listening to the preacher I have a bible open, scouring the focus passage and its surrounds. The preacher has their own interpretation to give and that's going in through the ears. Going in through the eyes at the same time is the text as I dip into some armchair exegesis.
I'm told it's a nasty habit because it looks like I've zoned out and am on twitter.
The truth is that it's a nasty habit because sometimes I find things that the preacher isn't talking about. That doesn't mean the preacher is wrong and must be corrected all over the internet, it just means I've seen something else in the same text.
Although maybe I'm the only person who does it during the sermon itself, I think that it's the right thing for all Christians to do. It's part of the refusal to check your brain at the door of the church when you come in - it's the commitment to intellectually embrace and wrestle with the faith and with the text. It's the commitment to refuse to be led astray by poor preaching. It's the commitment to refuse to mentally fall asleep while appearing to be listening. And last on my list, it's the active working out of Christianity as it critiques itself from within.