This is a good example of an animation mistake that didn't show up until final rendering. The grass in this shot is a billboard particle system. Normally, they are not supposed to move, and so the "Physics" simulation panel is set to "No". However, in this case, I mistakenly set it to "Newtonian", which apparently causes the grass to fall under the simulated force of gravity, causing a very disconcerting and obvious error in the final render.

However: there were no animation key frames at all in the scene file the grass is taken from (it's defined in a background set); the grass does not appear at all in the GL pre-visualization renders; and the movement was really not obvious in the handful of check renders I ran prior to rendering the shot.

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