For anyone who has sometimes noticed an impromptu slideshow not starting on the selected image, it turns out that this is a reproducible bug. Oddly enough, it depends on the number of images in the folder and what image number you try to start the slideshow on.
Example:
1) Create a folder with 174 images in it
2) Click on the 4th image in the filmstrip
3) Press Control-Enter
Actual Result: The slideshow starts with the 3rd image in the filmstrip.
Expected Result: The slideshow will start with the 4th (selected) image in the filmstrip.
Another Example:
1) Create a folder with 376 images in it
2) Click on any one of the following image numbers: 99, 102, 109, 116 and 123
3) Press Control-Enter
Actual Result: Slideshow starts on image prior to the selected image.
Expected Result: The slideshow will start on the selected image.
Comment: Bug is present in LR3 and LR4, including 4.1 RC.
