Enemy fire will make even the hardiest troops waver if their comrades start to fall in great numbers. Outright flight is unlikely and unwise in the face of intense opposition but even the most seasoned veterans may fall back to regroup and reconsider their tactics if they suffer too many casualties.
In Warhammer 40,000 such events are represented by units taking Morale checks when they suffer casualties in the Shooting phase. Any unit losing 25% or more casualties from its current strength to shooting during a shooting phase must make a Leadership test to hold their ground. Morale checks, falling back and regrouping are covered in more detail later.