Usually nothing will happen to you if you ate some bread without noticing the bit of mold on the bread but if you continue to eat moldy bread or food then it could lead to food poisoning due to toxins in the mold.
As long as you're not constantly eating moldy bread or moldy food then you should have no health problems as a result and shouldn't get sick because the acid in your stomach should break down and kill off the little bit of bacteria that was contained in the mold you ate.
If you continue to eat moldy bread or moldy food then you could get an illness which can result in stomach cramps, fever, diarrhea, vomiting and nausea.