I found that when a Zazzle product in the Nifty Slideshow Sharer used an asterisk in its title, it was causing an error deep in the sharer's guts when a new search was being done.
Now fixed :)
I've also made sure that a search from a shared slideshow now uses a representative image from the new search and not the original slideshow (as was happening before). It was ok everywhere else but not Facebook nor Twitter.