This might have been pointed out before, but during Unforeseen Consequences there’s a scientist hiding in a bin.
When you go up to him, this sequence starts:
“Gordon? Is that you? Is it saf-ARGH!”
headcrab attacks
“Oh dear, I appear to have soiled myself.”
You can kill him with a grenade or a gun if you like, but interestingly if you kill the headcrab with a grenade without killing the scientist, he completes the first line of dialogue and then nothing else happens.
Just thought it was a cool little detail, if intentional. 
