Nonsense everybody did at least one stupid thing on their first playthrough.
Probably, but let me look at some 'stupid' things people have done.
Stimpaks? I played Starcraft. I know what they are, and I also know how to read. They clearly state +XX amount of health. That's a good thing.
Used the bat for a while? I knew I had a 10mm as soon as Amada gave it to me. Again, I posses the reading ability, which most people don't seem to have.
Killed Caravans? I can understand how other people would make this mistake. Ugly mutant cowthing... But I never opened fire on it.
Let Lucas Simms die? Sometimes I do that on purpose so I can get his clothing and house key.
Walking straight into paradise falls? I barely touch it on a normal playthrough. Went there once when I went Behemoth hunting for the achievement. Since I
know how to play the game, I did fine.
Side note; I am a very defensive player anyway. I tend to survive through things other people wouldn't.
Using a chinese (or otherwise inferior) weapon? Never did, I look at the damage. Same thing on Oblivion.
Getting owned by bloatflies? Not sure how someone could get killed by one of those things. They're practically 1-shottable.
Couldn't find Megaton? How? There's a massive yellow sign with an arrow pointing towards it barely two steps out of the Vault. Miss that, you still have a map marker.
Taking bad perks? It's hard to fail with most of them, but again, I have a lot of RPG experience so I allocated stats efficiently.
Neglecting the Speech skill? Okay, I'll give you that one. It has a bigger role in FO3 than in any of the TES games. But there's an alternative for everything.
Taking a teddy bear out of a shopping cart? Didn't come across this particular trigger until I was purposefully hunting behemoths. Besides, I don't pick up everything I see.
Drinking all Quantams? I didn't know about the quest either, but I generally avoided anything with +radiation. Though it turns out it doesn't get debilitating until much later. As a general rule, I avoid any radiation whatsoever.
Hiding behind cars? I only had to see one explode to figure "Cars. Bad Cover. Got it." To be fair, I still get hit with some of its splash damage but give me some credit. Those explosions have a hell of a range.
Second side note: I don't blame anyone for falling under any of these. Learning curve and all that. Just simply defending my intelligence
Took the 30% xp perk? Hey, not a complete waste. Well, less of a waste when you get the 10 extra levels.
Ran out of bullets (due to not knowing vendors sold some)? Also not anyone's fault. It happens. Not to me, of course. My experience from Oblivion showed me that resting 3 days, shops restock. I would just sell loot, buy ammo to balance. Wait 3 days, rinse repeat. Hell, my common strategy is to explore and get at least a couple other 'cities' so I can fast travel and do the ammo-dance several times until I have no more loot to sell.
Tried talking to everyone including raiders and muties? The fact they shoot at me and scream insults kind of tells me they don't have a dialog option.
Kept drinking/eating, not knowing about Rads? Again, my effective READING ability comes into play when I see that big red
+2 Rad /sec thing.
Getting lost in buildings? Not a 'stupid' mistake for people. Some of those things are hella hard to navigate.
Didn't know what skill books/bobbles were for? Again, experience from TES came into play. They also explicitly state "You gained +10 (+1 for books) in a skill". Again, I know how to read.
Sold the Lincoln's Repeater? Hell, I never even knew about it until I looked up unique (IE: Best) guns.
Stepping on traps? Fine, you got me. Still don't think that's a 'stupid' mistake for anyone. They're tricky to see, which is kind of the point. Mines however, I'm pretty good with.
Not knowing about the Fatman? At GNR, it gives you 8 mini nukes. 250caps (Around 150 with low barter) a pop? I wouldn't use it if I wanted too. That's way too much cash to pass up. I kill the behemoth with an assault rifle.
Using VATS for everything? Viable tactic, if a bit slow and tedious. I don't use it all the time myself.
Taking the Here and Now perk? Not a complete waste, but only if you leveled from killing a mob, so you don't lose any experience that way. Also effective with the 30% perk, so you could gain a level and a half.
Old Olney. hooooo boy, that's a big one. But never a 'stupid' move for anyone. But players quickly learn to fear that place.
Not knowing about the light on the pip-boy? You got me for a second time, it took me a while. But that's never made a very obvious control, either. I discovered it while being annoyed with the delay of my character bringing up the Pip-Boy menu, holding down the button and thought, "Who, light. What...?"
Anything involving killing an NPC that isn't generally an 'enemy'. Honestly, you're all idiots.
Not knowing about the Wait function? Again, TES player here.
Bloody Mess perk? Never bothered, it's a 'fun' perk more than anything.
Using up Ammo? Not a stupid thing. In my first playthrough, my 'learning' phase. I was very conservative. I also already mentioned I'm a very defensive player (big on thief/assassins in RPGs)
I got to page 5 and things started repeating. So, there you go.
Every action you all consider 'stupid' was mostly averted because I know how to read.