In the Half-Life 2 series of games, if you weren't careful to attack a zombie's headcrab, it sometimes would pop off the dead zombie and attack you. Is that something I should worry about in Black Mesa: Source?
Yes, it is something to be aware of. I don't have any proof beyond my own experiences that this happens though.
One way to mitigate this is to aim for the head. Killing the head crab also kills the host, and there is no risk of the head crab jumping off.