Kate Wing nails it:
Scientists - If you have to preface your slide with “you won’t be able to see this” it shouldn't be a slide.
I have often heard some variation of, “This slide isn’t very clear, but...”. If you know that, then why are you forcing me to look at it? Apologizing for a slide might have been acceptable in the days of 35 mm film, where you couldn’t see the results until the film was actually developed. But we are living in the digital age, where high quality previews are immediate and photo editing software is everywhere.
You should always show the best image possible. Sometimes, that best image might not be so hot, but you should say, “This is the best available image.” Because that tells the audience that you respect their attention, and you put in your best effort to track down or make the clearest graphic you could.
Another phrase to listen for is, “This slide is to remind me to tell you...”
No! Slides are not for reminding yourself of what you want to say. Notes are to remind you what you want to say. Teleprompters are to remind you what you want to say. Rehearsal is to make it so you don’t need reminders at all.
If you hear phrases like these coming out of your mouth, you know it’s time to change your talk.
Never apologize, never explain.
Picture by Andrew Coulter Enright on Flickr; used under a Creative Commons license.