Under the condition of strenuous exercise with inadequate oxygen supply. ;D
When your muscle cells are asked to perform work at an easy relaxed pace they take in glucose, fructose or sucrose plus oxygen from the blood, and using the "Aerobic Respiration" method, they generate cellular motion energy without releasing lactic acid.
Read more about that amazing process here:
HOWEVER when your muscle cells are asked to perform strenuous work at an over-extended pace, (like your running away from a bear who wants to eat you). Then the oxygen becomes in short supply. There is not enough oxygen supplied by the lungs to increase output under the "Aerobic Respiration" method.
So, the muscle cells, knowing that they will die if they don't produce more energy, have a Plan - B. They can create massive amounts of energy without oxygen with the drawback of creating lactic acid. But that's fine, they will clean up that mess later. They start using an "Anaerobic Respiration" method.
Read more about that here.
The muscle prefers to make energy using "aerobic methods", but in extreme situations, your muscle cells have a "turbo" option: "Anaerobic_respiration".
This Scientific American Article answers your question:
And read this:
Your muscle cells will undergo lactic-acid fermentation (Creating cellular energy anaerobically) when there is not enough oxygen in the blood to create cellular energy in the preferred aerobic way.