You mistakenly cooked chicken breasts in crockpot for 8 hrs on Warm instead of Low Is it safe to now cook for couple hrs on high crockpot after only being on warm all day?

This is a tough one to answer. It might be OK or it might not. That depends how hot it got on 'warm' and what kind of bacteria were on the food. I have a crockpot that the warm setting is no different from low, but I don't know how your crockpot's settings work. It may also depend on the recipe and how you prepped it.

No one can tell you if it is safe. You'll have to make that call yourself.