Noticed this when using a head model with a thinner neck.
The headless texture has visible bloodstains baked into the texture, which becomes visible:

This probably needs another approach as for setting the player/diffuse.png texture whilst the player is alive; considering now that heads can now be changed independent of the model, the playermodel is always one of either:

• "actors/player/female/headless"
• "actors/player/male/headless"
  

  actors/player/player.cfg

  Lines 137 to 141 in 0da6da6

  	if (|| [strcmp (mdlname) "actors/player/female/headless"] [strcmp (mdlname) "actors/player/male/headless"]) [
  	iqmskin * "<dds>headless.png" "<dds>mask.png" 0.25 0.0 1 0
  	] [
  	iqmskin * "<dds>diffuse.png" "<dds>mask.png" 0.25 0.0 1 0
  	]