Disclaimer: Well, perhaps not all ISTJs are like this. But it's true for me at least, most of the time. It may even be just me, not an ISTJ thing.
When I'm engrossed in something, like when I'm actually busy on the computer, nothing goes in if you try to tell me something. I'd pay lip service a bit, but I'm really not paying attention. So please don't go mad later when I say, "I don't remember you saying that!"