Mine was playing Zyra a ton, I read her description on the first day I played with the free to play champs, I thought her seeds instant killed people if you stepped on them. "If an enemy steps on a seed, it dies."
A shitty short distance move on a 5 minute cooldown? F*ck that.
Yes, finally, the ignite upgrade! 200 damage at level one is gonna be awesome for winning my lane!
head is his nose
Maybe not that dumb, but I build Armor against Kat, because she uses blades, and MR against Caster Minions, because they use wands.
Oh and I build AD on Alistar, because there is nothing magical about a bull pulverizing you. That one was dumb, because it's Ability Power, not Magical Power.
Ranked was a more serious game mode where people played smarter and didn't feed.
My friends described Teemo to me (very poorly) before I started playing.
What I expected from their description was a very scrawny witch doctor character with a blow gun, who functioned in game as a sort of sniper.
I was a bit surprised when Teemo ended up being a Gerbil with a pink scarf.
I thought that Ziggs had a death passive for the longest time. His base skin's death animation has him zip around in the air and explode, and I thought that my champion could actually be damaged by it.
Haha, I can't think of anything for League atm (having played some Dota 2 beta I was somewhat prepared), but I do remember a similar silly misinterpretation I had with an Armored Core game; the description of the rocket weapon stated that it "Sets target on fire". For some unfathomable reason I twisted it into meaning that when I fire it it sets the target (locking onto it). I couldn't understand why it just went straight XD
I assumed that the most expensive item at the shop was the best one to get all the time so I just bought 
on every champ that i played
I went armor Rammus mid against Kata. SHE HAS KNIFES OK? Also I didn't know I could stop autos or deactivate them so I just bought attack speed to randomly last hit more. I assumed everyone was infinitely better than me so I stuck to customs with 9 bots for like 50 games. I didn't know what that green zero on my abilities was so I just built ad.
I for whatever reason thought buying a Nashor's on Swain would increase the attack speed of his birds while ulting...
The game is balance-able by riot staff.
I thought if you got a Zilean bomb on you, you could run at him and damage him with the blast.
I also used to build Garen full tank. He was my 1st main when I started in late season 3. I used my full tank build until close to the juggernaut update in season 5. The scariest part was that I actually carried games with that build.
I remember just starting a match without any gamd knowledge, picking nautilus (because he looked cool) to botlane, with surge and that ability that buffed your minions.
I also remember buying shoes first item because i saw a picture of rammus and cass putting shoes on their heads.
I used to think that the community was decent. BOY WAS I WRONG
"I think I'll do alright with Singed..."
I had a bunch of screw ups when I first started League back in Season 2 in my home country.
When I started playing League, the first character I used is Shen because he was a ninja so I thought he was an assassin. Turns out, he's a tank. Boy was I so wrong about him.
Also back then, I used to run Ghost on Yasuo because I assumed his E was too clunky and unreliable to use at first glance.
I used to think boots were literally useless. I thought the 20 movespeed wasn't good for anything because I didn't go super sonic speed after buying them.
I use to think 
was a tree back before his rework
I thought if you built AP it would increase you abilities damage no matter what, so I built AP on everything to try and one shot people.
Just built full AD on everyone. first champ i ever mained was Veigar.
I used to main ADC (now main top) when I started and would flame my supports for having no cs lol.
I thought that I could play any champ I found fun and have a reasonable chance at performing well and winning. Boy was I wrong there.
In my first couple bots games of all time I built ability power in ashe. since obviously i wanted my abilities to do more damage
That for example, B.F Sword actually lowered the cost of Infinity Edge not build into it with the full price.
Also, Ghost doesn't make me invisible?
I bought attack damage on everyone because you need ATTACK DAMAGE for ATTACKS to do damage.
I literally thought Singed was a pickle during my first week of playing.
"Janna's cheap, she's probably low tier"
I thought 
Spin2Win worked on structures.
"Why would I spend IP on runes, those stats are way too low to even matter" Also; "... Eh. I'll deal with masteries later. Too many numbers to care."
I could mention the "Smite champions" thing too, but I'm pretty sure everyone did that.
Very first game I played 
and rushed 
Why? Because it stacks infinitely so by late game I will be UNKILLABLE
I thought 
was a good champion because I played him whilst he was free in ARAM and did very well. ARAM doesn't translate to normal gameplay lol
I took heal in every lane I took, ever. "yo whats this shitty teleport garbage who would use this? im just gonna go heal fiora top" "taliyah mid is spicy why do these people tell me to take ignite instead of heal? that shits bad" Real conversation with my brother "I want to play Kha'zix jungle." "You need to have smite." "What's smite? I don't need it just let me play Kha'zix." I thought Fiora was Fiona for a while.
The whole AD vs AP confused me for a long time. Assumed AD for auto attacks, AP for abilities (quite sensible I think).
I also somehow got round to thinking that Udyr was a crazy intensive mana user...to the point that all the items I bought were based on whether it had mana or not. Created some pretty unique builds...
I always rushed a face of the mountain on zed when i was beginning because i was always behind on gold and i saw it gave gold. i never knew you could kill minions for more gold
I though Master Yi was an adc cause he attacked things alot.
