In certain circumstances, even a single pull pin can prove to be excessively safe. All down to training - or lack of... Consider the following by Eugene Sledge, 1 US Marine Div.., Okinawa, 1945 (from his book "With the Old Breed - at Peleliu and Okinawa", ISBN 0-19-506714-2):
"... On the right we began to hear American grenades exploding well within our lines... ...Then the word came along in the dark to be sure all new replacements knew exactly how to use grenades properly. One of our new men had been discovered removing each grenade canister from a box of grenades, pulling the sealing tape from the canister, and then throwing the unopened canister at the enemy. The Japanese opened each canister, took out the grenade, pulled the pin, and threw the deadly "pineapple" back at us. The veterans around me were amazed to find out what had happened. The incident, however, was just one of many examples of the poor state of combat readiness of the latest group of new replacements."
Tom.