My cookies flattened out in the oven instead of staying balls. What did I do wrong? I measured everything out by weight and the oven was 350

In addition to the points BerryBaby laid out, inaccuracy can also be introduced during the browning of the butter (how much water do you evaporate out?), how much you ground the pecans, salted butter vs unsalted... Have you measured your oven accurately with a thermometer? Ovens usually do not run exactly at the temperature you set it to. Different areas of your oven vary drastically in temperature as well. Finally, trouble can arise if you run a convection oven for a recipe developed for a regular oven.

A number of factors could cause this. Dough was too warm, flour wasn't measured accurately, cookies sheets were buttered and should not have been, extra large eggs were used instead of large eggs. I place my dough in the refrigerator after forming it into balls or shapes. I always do this with shortbread. Keeps the edges from spreading. Hope these help.
BB💐