One recommendation I often hear is, "Never apologize for a bad slide." Yet many of my colleagues seem not to have ever heard this one. They continue to put up slides with long tables filled with data in font so small that it's impossible to read even from the front row. These presenters apologize for the busy slide, then say that they just want to point out one or two data points and that the audience should just ignore the rest.
Well then why not just create a slide with those one or two important data points in large font? It's easy enough in PowerPoint. The audience will be able to read the values and can concentrate on them. Better yet, add a graphic or photo that emphasizes the relevance of those data to the point you are trying to make. See the following example.
Much more effective than a dense table. So why do people persist in presenting bad slides? Part of the reason is sheer laziness. They cut and paste the table, graph, or diagram from one of their papers (or someone else's paper). They think that to do anything else requires more time and effort (which is usually not true). The other reason is that they collected all those other data and so must show it....right? Wrong. The audience will be aware of the effort you went to to get to that key data point. Showing extraneous data has the opposite effect of annoying, rather than enlightening your audience.
Showing a slide that is impossible to read or understand is also insulting to the audience. It sends a clear message that you don't care about them. And if you apologize, it means that you are well aware of the poor quality of the slide but didn't care enough about the audience to fix it.
The advice about never apologizing for a bad slide suggests that it's OK to use bad slides as long as you don't acknowledge it. Actually, most of us realize that this advice means never use a bad slide. No slide is better than a bad slide. However, it's easy for novices to misinterpret this advice....which is why I bring it up. I should mention here that using tables and complicated graphs or diagrams in posters is OK (because the viewer has the time to digest them and likely wants to see the data), but you should still design them well so that your point is clearly made.
I would amend this recommendation to be, "You should never HAVE to apologize for a bad slide. If, while practicing your presentation, you find yourself saying, "Now, I realize you can't see the data on this slide, but I just want you to focus on this number...", delete that slide and create a new one. Your audience will thank you.