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In Call of Duty 4: Modern Warfare. Notice the error message pop up, "dobj for xmodel 'body_mp_arab_ regular_assault' has more than 128 bones (see console for details)"
(Edited for Anonymity)
In Call of Duty World at War. Notice the error message pop up, "UNLOCALIZED(cannot spawn turret '30cal_bipod_crouch' which has a script, unless one instance placed in map)
In Call of Duty: Modern Warfare 2. Notice the error message pop up, "Cilent command overflow"