Do I have to say I pull the safety pin?

Imagine this:
You are in the middle of a heated session, in the midst of a battle, bullets are flying over the heads of your team.
"What do you do?!" - Your GM demands an answer right here, right now!
"I throw a grenade towards the enemy!" - You throw back a quick answer.
...And then the silence falls. Your team looks at you in disbelief and GM smirks when he describes what is happening.
"You pull out the grenade and throw it. You hit just the spot and the grenade just... lies there. You have never said you pulled out the safety pin."

Problem

For some players this is the essence of what role playing is. "Unless you say it, it did not happen."
And that's perfectly fine, as long as everyone is fine with it and as long as everyone has similar level of expertise in the area.
Now imagine that situation above happened to a player, who, while having some experience in gaming, wanted to try something new. "I have always played a medic, now I want to play a commando!"
And then... this.
What would this make a new player such as this one think? Quite possible options are: "I have failed my team." "I'm so stupid, everyone knows I should have pulled the pin."

This is, actually, a pretty complex situation on two levels - one level is 'what happened in game' while the other level is 'what happens between the players in the room'.

In game, this situation can be salvaged... or not. If the GM set this to be a very difficult encounter (or the players made too many mistakes) this single action could be the straw which broke the camel's back, thus, dooming the team. So what should a GM do? Should GM be a cold processor of the reality, or should GM take into account the fact there is a newbie on the team? A lot depends on the expectations of all the players and their social contract, be it defined or not.

In the real world, everything depends on the players. No matter what happened up to this point, this player will be remembered as 'the one who did not pull the pin in critical situation'. No matter if this kills the team or not, the player is under great social pressure from now on. If the other players treat the unfortunate newbie as a rookie and let them learn, this is not a major problem, but if they act condescending, patronizing, with anger or scolding... let's put it this way: do you think, that this newbie will try something new again in the near future? Or will the newbie stick to the role they are at least semi-competent with?

The good

I'm not saying such approach can't be fun - it can. Combination of fast actions and adrenaline with high stakes makes a very exciting session. On top of that, if you want to learn about things, making them part of your game can actually give you some incentive to study.
If you know a lot about being a commando and can tell different weapons apart and know at least basic rules of how to act in combat, then playing a commando will be fun for you. Also, in a way competing with your teammates can be quite satisfactory. Also, there is a lot of potential for "remember how I forgot to pull that safety pin? Luckily, Tom managed to save the day" kind of memories.

The bad

The problem here is that this can happen only in a very friendly team, where everyone is on the same level.
The situation described in the beginning has really happened.
The player, who was always a medic, wanted to try something else. She forgot to pull the safety pin in a heated situation. Her GM and her team stated it was an error and - in a way - punished character for the lack of player knowledge. That player went back to being a medic, thinking that being a commando is not her thing.
She even claimed that it was correct handling of the situation, but to me it seemed like she has learnt to think this way - however I cannot be sure, I never got a chance to ask.

What to do?

When is the situation above a good part of a gaming session? If our social contract includes lots of 'player knowledge is character knowledge' things, the approach above is a correct one. If during your games you want to mimic reality as close as possible, it is all right. If everyone agrees that is the best way to handle such situation, it is fine. However, this only works, if team members are on the same level of expertise and if GM has enough expertise in all the areas (or is easily convincable).

But... What if players want to try something new? Something outside of their area of expertise? What if a complete pacifist IRL wants to try playing commando? Or someone terribly shy wants to play a bard?

Imagine yourself playing a commando, a doctor, scout, bard or whatever that you have little knowledge about.

You know that commando training must have included bazilion types of weapons and ways to kill people, even if you personally can't even hit a mosquito when it sits on you to feast.
Doctor must have learnt about anatomy, even if you can't point to where your appendix is.
Scout probably should be able to stealth through various areas, even if comparing you to an elephant in china store is actually harming for the poor beast.
Bard would have immense knowledge about composing, making his own instruments, should be able to sing... you get my drift.

Our solution to the problem is quite simple and it still leaves some space for "you did not pull the safety pin", however applied differently.
What we do is we assume, that character is good at whatever it is supposed to be good at.
Hence, elf would have an easier time taming some animals, doctor will know how to sew wounds together, commando will know to pull the pin before throwing the damned grenade.
We do not contest level of detail of our narration. What we DO contest is our intention - what we want to happen. And we negotiate what will happen if we succeed and what if we fail.

Let's say I'm this commado, ducking behind the cover and wanting to throw a grenade to handle baddies that are about to massacre my team.

Let's say we negotiated that in case of success, I dispose of the enemy pinning the team down, but in case of a failure, I will put my team in a worse tactical situation. Now I know my edge cases, my best and worst possible outcome. So the actual result of my action will fall somewhere in between those two options, with critical success or failure can only magnify negotiated results.

And the stakes are negotiated before the test is done. Thus, if the test could potentially result in an unfair outcome a player can protest before the roll. 

But let's assume I accept the stakes.

I roll a die, pull a card, or <insert your way of conflict solving here> and I end up with a fumble (critical failure).
NOW this failure can be narrated as "You throw the grenade and as it leaves your palm, you notice safety pin glittering in the light. You hear the enemy duck in panic, when the grenade lands, but they quickly realize what has happened. Now your enemy has a grenade, which they did not have before."

The result is the same, but this time the potential results of the resolution were well known before the test (roll, card draw) was done; and the actual RESULT in the game world came from the INTENTION and the TEST, not from raw player declaration.

What is important, we know the result - we know that somehow the enemy team got stronger. We don't know how it got stronger or what happened, but this can be generated to match the resolution. And thus, interpretation was put in the game as 'grenade did not have the safety pin taken out and enemy soldier could pick it up'.

Interesting? I certainly hope so. But what if it is just a failure?

Maybe you timed it badly, so the explosion leads to your team being more exposed. Maybe you simply wasted your resource (grenade) and you will not have a chance to resupply later? Still - those stakes are selected before the roll (or card draw).

And what if it was a success?

You simply disposed of your enemy in this spot and can move attention elsewhere.

And critical success?

Maybe your grenade led to another enemy group being exposed? Or maybe that rookie in the team accidentally threw a tear gas grenade instead of a regular one and now your team will get a very important prisoner? (You might ask why did he even take such grenade on this action - well, rookies are rookies, no one knows their logic ;-) )

Options are plentiful and the character is not being punished for the fact that player is inexperienced / tired / distracted.

Do I have to say I pull the pin?

Depends on you. On your playstyle. Social contract. What is fun for you.
Just keep in mind there are other ways than yours and all are valid.