There's probably a better way to explain this, but here goes. I have DV footage in a viewer - set an in and out - edit into a sequence. Everything is fine. Then I render that section (it's being inserted into an HD timeline, yes painful).
BUT, after rendering the clip the footage changes to a different point in the original video segment that I lifted the clip from. For instance, I pull a 2 second clip from 50 seconds in and after rendering the clip now references a different portion of the video from 23 seconds. Weirdest thing I've ever encountered.
Same thing happens with a still image. I create a still image of a frame in the original video and once rendered or even exported, the resulting still frame is from a different point in the original video.
The Visual Rabbi
"Crafting Visual Messages to Engage and Persuade"