Star Trek II: The Wrath of Khan - Reliant's See-Through Warp Nacelles
After the Reliant's first surprise attack on the Enterprise, but before Kirk retaliates, there is a shot of the Reliant coming towards the screen. It appears that the warp nacelles (especially the starboard one on the left side of the screen) are partially transparent because one can see the background stars through them. The scene can be found at time index 52:02-52:05. I've also seen this special effects mistake on the original VCR version and a TV broadcast years ago. If you play the scene over and over, it's pretty obvious that the stars cannot be explained away as reflections. I'm not sure what the rating is, but I've certainly laughed at this one over the years.
**Contributed by: John Bogen
DVD Widescreen Collection, The Director's Edition