Recently I received a support request wherein the user was attempting to place 2 "This Week in Pictures" web parts to a page. There could be various reasons why one may want to use multiple picture libraries on a page; like for e.g. if a department site is publishing award recognitions for team members for successive operational periods, then they may retain the previous period and the current period as two completely different picture libraries. This will give all site visitors the chance to view 2 picture libraries.
The issue was that when you view these libraries in "Slide Show" mode, you would see the same set of pictures in both the slide shows! Weird, isn' t it!! I too felt the same!!!
One thing was sure that somehow when the slide were being rendered on the browser it was pointing to the same set of pictures!
Initially I thought that maybe both the web parts were accidentally linked to the same picture library and that this may be a user error due to incorrect setup. But that proved wrong as soon as I reviewed the web parts properties and found that they had been correctly setup. After spending some time trying to look fo reasons it struck me that the browser was somehow getting an incorrect reference to the same library. Upon further investigation I found that (and this will be interesting to some of you who like to write application code) the Javascript that the browser emits for the page had a hard-coded link to the picture library; whereas this link should have been dynamically populating the URL to the respective picture libraries. Yes that was a bug in the code!