I was playing around in the film viewer, setting up a camera view. In the process of getting the angle I want, I played the shot several times.
The shot (no real secret, won't spoil the video) is a close up of a P-39D's exhaust pipes as the engine starts up. The shot includes the puffs of smoke that accompany the engine startup.
Here's the problem:
1) I can play the scene fine by manually clicking the "play" button and see all the smoke.
2) When I click the camera views "Play Shot" button it plays with no smoke.
In the camera view you can see smoke behind the tail of the plane (just barely) from either 1) the other side's exhaust, or 2) this exhaust is being displayed "behind" the airframe.
I'm not sure why the camera setup (identical angle, position, etc) is monkeying with the little effects like this, I figure it's a bug worth reporting.