I noticed that if i just click on next image (does not matter where in nemu, next to the picture) sometimes (10% reproduction ratio), typicaly when image size is changed from vertical to horizontal (tested with same image height) next image is not shown. Just narrow white bar that supposed to be a background. Then, I need to get back to thumbnail view to be able to see images again.
you can reproduce it it here:
http://raffoto.pl
go to New York gallery and just click on next with random intervals. You will encounter the problem quite soon.
http://yfrog.com/12galleryerrorp - bug illustration.