Ok I realize this won't fix every broken sandal, but if you have a rubber or plastic flip flop or sandal where a strap has broken, this might work.
The first example is where the center strap tab broke off the bottom.
I literally just used a screw to screw it back together. Ok, I did screw it in, unscrew it, add superglue to the twists of the screw then screwed it back in. Thought that would help it hold.
It has even held through four wears already! Quick and easy fix!
Another example is where a side strap broke on the top. Again I just used a screw to secure it to the sole. I put super glue on the screw portion on this one as well. Can't even feel it when I wear it.
And since my foot is on it when I wear it, you can't even see it. Hope this helps you salvage your favorite pair of flip flops :)