Been a while since I've seen E.T., but chances are that she was eating the hamburger (note that they don't have to show her on camera doing this; she can be eating the hamburger while other characters are speaking, etc). In that case, the hamburger would tend to diminish in size over time.
What he means is that from shot to shot, the hamburger is half-gone, almost gone, half-gone again, almost gone again. it was a continuity problem, common when a scene is shot over several hours (after all, they wouldn't have made Gertie eat the same cold hamburger throughout an entire day of taping). This is indeed a legit slipup. It's even documented in one of the Film Flubs books (I'm not sure which one, I have them all).
When they have a scene with food, it does change size because its food, its not like anything else, it goes away after awhile, This still is a slip up. but when they show scene with food, a lot of the time it will keep changing sizes, it isn't really that much of a big deal now is it? I didn't think so.
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