When I read about someone putting the pin in a dropped grenade, I think “Why is he doing that? He’s going to die! No, this didn’t happen, it’s just made up by a hack who doesn’t know how grenades work.” Congratulations, you just destroyed the immersion in your story. Similarly for a grenade exploding while still held pending the throw, or a thrown grenade with the spoon still against it (for graphical media).

No need to trek through the snow to the library, research is now as simple as visiting http://en.wikipedia.org/wiki/Hand_grenade from the comfort of home, yet curiously, this seems to be a disease of newer works, despite older works having hand grenades in their stories if anything more often.

So, evidently there’s a need for a primer for writers. Here goes:
Anatomy of a hand grenade: The body of a grenade is a shell containing explosives and a timed fuze. Against the shell is a lever called the spoon, which is connected to a spring-loaded trigger called the striker, which starts the fuze. A safety pin holds the spoon in place.
Operation: Hold the grenade in your throwing arm, pressing the spoon against the body. Pull the pin. Throw the grenade at the enemy. With the pin removed, the spring will now push the spoon away and the striker starts the fuze, which in a short time will detonate the explosive.

Note that the fuze can’t be stopped, and it’s the spoon which triggers the grenade, not the pin. So a held grenade can be re-safed by replacing the pin, which might be the seed of the myth that the pin will deactivate a cooking grenade.