I forgot to reserve pasta water! What can I do???

I keep leftover mashed potatoes frozen in 1-Table spoon amounts to thicken soups and sauces. If you haven't any, microwave a small potato (I like to wrap them in a wet, clean towel or several layers of wet paper towels. Don't forget to poke holes in it!). Then you can purée it in a food processor and then add it to the sauce.

A spoonful of bread crumbs also will work wonders to thicken a too thin pasta sauce

Is it for the sauce? You could use regular water, after you add your pasta to the sauce and cook for a short time it will thicken up.