Well, to answer your question, throughout all the times I've seen Raiders I've never seen this aspect change.
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I think it could've been something with your tape, however, because after scouring YouTube for a version of the opening scene from the 99 VHS, there was no aspect ratio change.
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Odd. I've never heard of (or seen) this happen before on any version of Raiders that I've watched (even the 1999 version where the aspect ratio changed for you).
New theory: Perhaps it was your TV that contributed to the effect, as none of the VHS tapes I've seen have this particular feature.
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Thanks for the info. It does help a great deal, actually, and has helped me narrow down the aspect change to a couple of reasons:
1. Some of the "Complete Adventures" sets had short little ads playing before the film started. If these had the stereotypical "black bars" seen with widescreen, perhaps a technical screwup caused them to remain in the first few minutes of Raiders. This seems less likely then my second theory, which I think is most likely the correct one.
2. Your TV screen was adjusting itself to accommodate for the film. If the transition looked more mechanical (which is what I derived from your description), it's most likely that your TV and the tape might have been adjusting to allow the video to be played.