When the page loads, the video thumbnails show for just a split second, then disappear -- replaced by black boxes with a white play button. (There's one exception -- the third video from the top does show the correct thumbnail). SEE SCREENSHOT.
This happens when viewing the page on my iPhone 4S. Not sure about Android devices or other iPhone models. Doesn't happen when viewing the site in any desktop browsers. Leads me to believe it's related to how YouTube handles video on mobile devices.
Can't figure out why this started happening. Used to work fine. From what I can tell, YouTube made a change to the way the iframe works -- must have been in the last couple of months. Not only did the thumbnails start acting up about that time, but the rounded corners (border-radius) CSS I was using on the YouTube iframes has also stopped working in Chrome AND has caused YouTube videos to not play in FireFox for certain people. (Removing the CSS fixed the issue of videos not working in Firefox, but does NOT fix the thumbnail issue).