The two cases for this are a zero regen shield (eg the superconducting shield) that has run out of recharge items, or a non-superconducting ammo-using shield in 1.9.2 and previous versions (which cannot use its shield regen items because of the AI hardcoding). In these cases the ship will run from combat forever, which may lead to a memory leak as the ship strays further and further out of the normal playable area.