Right! So weird.

I believe that's a black swallowtail egg.
http://ragamuffin.com/butterflies.htm

Butterfly eggs are tiny. The white stuff makes it look like some sort of poop: lizard, gecko, snake, bird?

Watch & see if flies come to eat it...

Here's something from UF on Giant Swallowtail eggs. I'm sticking with that. I don't know of any other butterfly that lays a single egg on top of a leaf.

http://entnemdept.ufl.edu/creatures/citrus/giantswallowtail.htm

I believe it is a beetle egg, but I don't know which beetle. Beetles are known for laying yellow, oval eggs (but usually in clusters, not singles). Perhaps she was "interrupted", LOL.

Carol in Jacksonville

FWIW, I have giant Swallowtails here & have personally seen the eggs - they are a fraction of the size of that shown in the photo - also round, not ovoid. Link above even says eggs are 1-1/1/2 mm. Photo shows something that looks to be 3/4 cm or so...