Using a 500D, with an autoexec from January 26th:
The magic zoom box alternates between visible and invisible at a rapid rate (flashing), perhaps 15 times per second or so. It's a bit distracting when you're trying to fine-tune the focus.
Similarly, the histogram alternates between an accurate presentation of the current scene, and that of some unknown older scene (possibly the first frame when LV was activated). The rate of this is much slower though, roughly 1 / second. Still, sometimes it may confuse you about which of the histograms is the correct one for the current scene.
Both 1) and 2) almost always happen when the movie mode is activated for the first time after turning on the camera. Switching to M (photo mode), and back to the movie mode fixes the problem, until the camera is restarted. The live view mode is restarted when doing this, which may have something to do with why the problem goes away.
Note: refreshing the GUI (MENU while recording) doesn't help with this.
- Previewing an option in LV (such as K white balance) using the ZoomIn button: this does not work while recording a movie. It works otherwise.