The little details in a presentation might not make or break its message, but they do count. A spotless presentation shows you made an effort, that you take your audience seriously.
The result of some maintenance work downstairs in the parking lot of my building
Here are some things to look out for: typos in common words (if your spell checker flags something, there is probably something wrong), typos in names of people, inconsistent use of fonts and/or colours, small misalignments of objects, a text wrap gone wrong, or making sure that the positions of titles on all slides is the correct. I remember the senior partner on my first McKinsey project coming into the graphics design room to hold all the paper slides against a strong light to check these things.
Yes, I know that I am contradicting myself now and then as I get reminders about typos in my blog posts...