The Previous button points back two images, while the Next button points to the current image.
This seems to only happen when entering browse by selecting one of the random images from the gallery home page.
Confirmed with php 4.3.9/apache 1.3.33, and when the links are clicked there is an error,
modules/coppermine/displayimage.php - Line: 517
Error occurs when a new image is added between when the page with the thumbnail is generated, and when the thumbnail is clicked. This affects the Next/Previous buttons as well. This is due to the fact that new images that are added "bump" into the front of the order, and the buttons and thumbnails reference images based on "Position" in the order. As such, if you were viewing image 4, it suddenly is 5, while image 2 becomes image 3. Thus, going "next" goes to image 5 (the same image again), and going back goes to Image 3 (previously Image 2)