When you first meet Lamar (Dr. Kliener's pet Head Crab),
enable the impulse 101 code and shoot her with any gun.
Lamar will instantly disappear. You will see some blood on the
floor after she disappears if you shoot her with your pistol.
Alyx, Barney, and Dr. Kliener will all act as if she is still
there. Only Barney and Dr. Kliener do not look on top of the cabinet
where she is supposed to be. They will look in completely different
directions. Lamar will later return in the Teleporter room. You
can shoot her again and the same thing will happen.

Glitch: Lamar passes through Dr. Kliener's
leg
When you and Alyx go to Dr. Kliener's lab via the Teleporter,
Alyx bangs on the door and asks Dr. Kliener to let you out. Dr.
Kliener then opens the door holding a shotgun. You will then see
Lamar appear at Dr. Kliener's feet. However, she passes through
Dr. Kliener's right leg in the process.

Glitch: Jumpy Dr. Kliener and shotgun;
invincible and vanishing Lamar
When you and Alyx go to Dr. Kliener's lab via the Teleporter,
enable the noclip code and go through the door into Dr.
Kliener's lab. You will see Dr. Kliener and Lamar behind the door;
both of them not moving and a shotgun lying of the floor next
to Dr. Kliener. When the moment arrives for Dr. Kliener to open
the door, he suddenly jumps towards the door to open it, and the
shotgun suddenly jumps into his hands. If you try to shoot Lamar,
she will not die. After Lamar runs out of Alyx's view, she just
disappears around the corner (and is not actually hiding at all).