In two webinars over the last two weeks I heard the main speaker asking the technical person/producer of the event which slide was showing on the screen because the main speaker couldn't tell what he was seeing versus what his audience was seeing. This situtation is easily remedied by having the lead speaker log into a second computer as a participant. By having two computers side by side, the speaker can always see the presenter view and the participant view simultaneously.
If you are striving for flawless delivery of a webinar, try following this best practice. I've used this technique for years, and time and time again it has been a life-saver.