This cartoon went through a bunch of different permutations before this final version. The “serendipity is up” was always in place, but it took forever to figure out the rest of the gag.
One of my tricks when I’m stuck like this is to start digging into various references like dictionaries, thesauruses and idiom guides.
As you can probably tell, the thesaurus saved the day on this one. Which was lucky for me.