I often work with webinar guest speakers who tell me there is no need for a rehearsal ahead of time. They know their subject, they have delivered the speech before, and they know how to click through the slides.
Then we get into the webinar and I find there are embedded assets on their PowerPoint slides that the web conferencing software can't display. Or they have active links on their slides that they want to use to show content (or to have the attendees click on to interact with web content). Or they planned to offer documents or spreadsheets for attendees to use in exercises. Or they forgot they have build animations and transitions in their slides. Or it turns out that they are used to PowerPoint not showing "hidden" slides in slideshow mode, but the conferencing software displays them.
There is never a good excuse for not running through all session content ahead of time inside the web conference room. Even if the presenter(s) don't deliver the full speech, you still want to confirm proper operation of all materials.
As an example, yesterday I discovered a WebEx limitation I hadn't been familiar with. I started up a WebEx Meeting with PowerPoint slides, a Word document, and a video clip. I validated proper operation and display of each piece of material, each in its own tab… ready for display in the session. I started my introduction as host and then passed presenter control to my guest speaker. Much to our mutual amazement, three different behaviors occurred for the three loaded materials:
1) The PowerPoint slides stayed visible and operable by the new presenter.
2) The Word document was still in its tab in the meeting room, but the new presenter could not select and display that tab.
3) The tab holding the uploaded video clip was instantly deleted.
I had to take back presenter control, show the Word document, and reload/display the video. There would be no way to guess these behaviors except by loading all assets and actually doing the operational steps of passing presenter control back and forth.
In an attempt to forestall the inevitable presenter objections to "wasting time on a rehearsal," I have stopped using that terminology. I now tell them we need a familiarization/refresher session on the conferencing technology. Or we need a technical runthrough. Even if they (and you) have used the technology in the past, you may not know when the vendor has updated it or there are changes to the way it interacts with updated third party software such as Flash Player, Java, PowerPoint, or web browsers.
Never assume your content will work properly. Validate every single presentation asset and every handoff inside your conferencing product. Every time.