If the visitor explicitly pauses an auto-play slideshow, then leaves and returns to the page, the slideshow resumes. Ideally, the slideshow would stay paused until the visitor explicitly re-activates the slideshow by clicking play.
Justification is that if a visitor does not like slideshows or finds they make the page unusable (accessibliity), they still will not want a slideshow on re-viewing the page.
Ideally, if a site has multiple slideshows, this would apply across all slideshows on the site.
This could be accomplished through HTML 5 local or session storage. It would not work on IE 7, but would cause no harm on it either.