Recently, while building a page full of audio snippets for a client, we ran into a show-stopping bug. The page was comprised of 12 blocks. Each block highlighted a voice artist and included a pure HTML5 player to allow users to hear voice samples. In the initial testing, we had just 3 of these voice artist blocks on the page. We had successful playback in Internet Explorer 8 and 9 as well as Chrome, Sarfari and Firefox.
Upon launch, when double digits of artists were loaded, the players stopped working in Internet Explorer. They still behaved perfectly in Chrome, Firefox, Safari and even Opera. Our first thought was to include some code to force the browser to render as IE9. No luck. After a good 15 hours of debugging, we finally discovered that Internet Explorer doesn't allow more than five HTML5 players per page.
It's not often that you find yourself in a situation where you need to have multiple players on a page, but if you do, remember to limit it to 5 for Internet Explorer.