Do not apologize for anything. Do not say “I am sorry this room is too warm”. This is not in your control. Apologizing for things outside your control is one thing, but apologizing for things that you have done is unacceptable. At a recent presentation, the speaker showed a graph measured over the last years from 1999. When the speaker finished explaining the data, he looked at the slide and said “Oh, the data for 2006 is missing. I better add that when I get back to my office”. That makes the audience feel unimportant.