a few different kind of low elo mistakes i see all the time
these are not in a specific order. not the best english incoming you have been warned. most of them i saw in vids, streams or in my own games on my lower elo smurf.
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(starting with one thats somewhat rare) not knowing if armor or magic pen. this mainly happened in urf but it also happens in SR. ppl play ap tryndamere or ad corki and they go voidstaff or last whisper. tryn does 100% physical damage, he has no magic damage in his entire kit. even if you go full ap, your e still does physical damage. magic pen has no impact. same for corki just the other way around. 90% of your kit does magic damage even your autoattacks do 80% magic damage. thus a void staff does way more for you.
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not knowing about "unique" passives. ezreal with
(spellblade passive)
some adc with 
(lifeline passive)
both of this sets share their UNIQUE passive. the game will automatically take the stronger effect. but you wont deal 200% bonus damage and slow. and you wont get 2 shields. -
not reading your champ spells. im gonna take my fave example that actually happened. 152k mastery mf "what my q can crit?" read your spells.
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not knowing "special" interactions with your main. almost 400k mastery zac, teammate tp's on one of his passive blops, zac goes "afk" and then asks in chat what just happened he thought he is dead anyways. yuumi refusing to w on twitch because "it gives twitchs position away" (it doesnt, yuumi also gets invis/stealth)
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mobafire/op.gg/whatever is not 100% correct. just because the highest winrate build on XY is to rush armor item XX first, doesnt mean thats the right thing to do when the enemys have 4 ap champs and their ad champ is 0/7. (you might thing this is obvious but you have no idea lol)
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not checking special item/spell interactions.
if you see duskblade above your spells this means the enemys dont see you.
if you stand in a bush and the buff disappears, that means the bush is warded. you dont need to sit in that bush for 30 seconds waiting to make a big play. -
not knowing cooldowns. i get it the game has over 150 champs and knowing the cooldown of all spells is insane. but at least know the cooldown of the champs that are most played in your elo. zoe e has a high cooldown and she just wasted it, you dont need to ping your jungler back and go "she will make you asleep under tower"
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this is a very specific one but i still feel like mentioning it. knowing when to buy
vs 
blood thirster heals for 20% of your AUTOATTACKS damage. if you are a champ that autoattacks many times, you get BT.
deaths dance heals for 15% of your damage dealt (this includes spells). if you are aatrox, deaths dance does way more for you as you deal insane damage with your q and thus also heal crazy damage.
8.2 is it an autoattack or a spell?
i see so many dravens go DD because "my q is a spell" well that is correct, but its a spell that makes your AUTOATTACKS stronger. thus BT is the better option (not saying DD on draven is wrong btw) another good one is garen vs teemo top. no need to use your q as garen when teemo blinded you. your q is a stronger (and way slower) autoattack. you will miss him and waste your q. activate your e and use your q once the blind is over.
those are all the things that come to my mind right now. hope it helps. these are the more "in depth" things that i see low elo players do wrong.
<>< here have a fish.
) is the vision denial item.
/
against sustain/heal comps or lanes.
Still going full damage Top/Jungle on Brawlers even when horrifically behind.
Not warding.
Using knockback skills at stupid times.
TPing to minions right next to the laner that just killed them when said enemy laner is still at full/almost full HP.
Going for Baron when they could end the game outright.
Calling for Baron when the only survivors on their team are low sustained damage champs (like a tank jung, a burst mage and their support).
