Have you heard about Hulu? It is a great service for watching movies and TV shows online for free (with a few ads though). The video player they use, however, has a rather nasty UI fail.
The player’s interface has two modes: when you don’t touch it, it displays a very thin progress indicator at the bottom:
And when you mouse over the player, it displays the extended UI:
Unfortunately, the progress indicator in the extended and in the default UI modes have different scale, so if you try to jump to a time point using the default UI but let your mouse loose just a little bit to trigger the extended UI, you end up jumping to a very different time point than you intended to.