So the time I felt ike the biggest noob was in world pvp.
In LK I had been playing my priest a lot and was chasing a mage who jumped off a cliff to get away. I jumped off the cliff to follow him, intending to use levitate and quickly realized I was not on my shadow priest. I was in fact on my Warlock. 
How about you?
opening this thread
In Wrath I jumped my druid off the big spire in Dalaran trying to see how low I could get before snapping into flight form at the last second.
Falling… falling… falling…
Click! Click! Click!
SPLAT! (Dead druid)
I had fallen into the “no flying” bubble.
I forgot to eat on a Pandaren character.
And trust me. Not eating on a Pandaren is very much a noob move.
In Vanilla I decked out my Warrior in of the Tiger Mail.
Back in MoP while I was solo farming the Warbringers in Kun-Lai Summit I moved my Lock’s Voidwalker to avoid the Vengeful Spirit since it was quite deadly at the time. It went after me instead so I moved myself away and had a lapse of environmental awareness and walked myself right off the cliff.
In Vanilla, while hanging out in Orgrimmar, I kept seeing combat/game text stating “Gamon has died”, so I tried sending Gamon a pm to find out what was going on.
A lot of falling deaths can be prevented with certain class abilities.
A lot of these abilities have cooldowns.
It’s not the fall that kills you…
Logging into the game for the very first time.
My noob moment was, when I accidentally killed a horde cow picking flowers, thinking that he was a mob. It ended up like this…
Lol! I don’t know why this had me laughing so hard but it did. Gj
Whoa.
May I suggest a snickers?
lol or a cupake 
running around the shrine in MoP on my mage to find the city portals
I was kind of new. I am in a guild and I am playing with my Druid. The guild chat was active and all.
I go for the first or second time in my life at the time to the mailbox and see the Hearthsteed. I picked the Hearthsteed but I thought it was of a guildie that missed it.
I asked in guild chat if the hearthsteed belonged to someone and the active Guild Chat went silent hard. Until a friend was nice enough to whisper me and explain me everything.
Questing in high mountain near the PvP area. I see a deer running so I attack it quick only to realize it was a Druid who promptly curb stomped me then laughed at me.
actually had a similar incident during BC on my main got zerged by 3 fel boars well I ran like mad toward this nearby camp figured the guards would pull some aggro and I was right they did sadly though they don’t take kindly to NEs running thru their camp silly me I failed to notice the big red banners hanging on the wall next to the doorway
when i got into my very first raid outside of LFR and im just here spazzing out irl sweating bricks and what not just tweaking out like i was nervous as shet.
pretty sure i was playing healer at the time but i put off raiding for a REALLY long time because i was scared now it’s like MEH i only get this way in PvP now but i already know i suck in PvP cause im more of a PvE player but i do like to PvP to break the cycle of mindlessly doing pve content
recent noobishness:
i switched raid frames last week. set it up to how I thought it would look and assumed it was serviceable. gallivanted off to heal the bajeezus outta the pandaland w-boss.
little did I know apparently there’s a setting I missed that allows me to actually click on a player from the frames! why that wasn’t default is beyond me buuuuuut long story short, I healed myself a lot before I realized what was going on. there was a lot of death- whoops. 
(perhaps the addon creator assumed we’d be using Clique along with it? idk but back to Grid2 for me)
just general refusal to use certain abilities/specs
I used to refuse to play arms cause it wasn’t as badass as fury