Most of the presentations I give, and receive, involve software development. More often than not this involves some amount of live coding by the presenter. As most developers seem to using Macs these days they're often using Keynote for any slides in their talk.
Keynote has a really handy presenters view that will show the current slide, next slide, and timing information on your laptop display while showing the presentation on the secondary display/projector.
Unfortunately, presenter view and live coding don't work well together. If you're using presenter view then your laptop and projector are showing different images. If you want your audience to see your live coding you need to place your editor on the secondary display/projector. This means that while you're typing you need to be looking at the projector's display, not the one in front of you. I've never seen this work out well.
No matter how easy you think it might be, typing while straining to look over your shoulder is distracting. It's bound to be a comedy of typing errors and comments about how hard it is to type on your laptop while looking over your shoulder. So just don't do it. If you plan to do any typing during your presentation then ditch the presenter view and mirror your screens. It'll save you some stress and keep your audience from losing interest in the guy who can't type.