add salt to a water/white sugar syrup. It turns into crack.
I didn't take a picture because I was too embarrassed. Just take my word for it!
*added later: I think I've figured it out. I added the salt once the sugar/water solution was boiling, AND I used fairly large-grained salt, which gave the sugar something to crystallize around. Is that a scientifically sound conclusion?
**added later still: It must have been the size of the salt crystals. Or the type of salt. Or I didn't cook the sugar/water enough. Or I boiled for too long. Because since this post, I've made caramel twice, and both involved adding fleur de sel at the end and...no crack! Just yummy, tasty caramel!