Top 5 mistakes you see people make

[Clathius]

The biggest mistake I often make is taking my actions before I remember to use my Hourglasses.    I want to kick myself when I eat something with my Oty and then tap my Hourglass and get a Graveyard.

[Kamietsu]

No cards left in deck. Uses hourglass.

[Tea is good]

Multiple mindflayers/ulitharids against a un-quinted oty and no aether quanta. happened to me :-( Always check your quantum supply people!

[XxDevilxX]

i once had to painfully lose on an ez win cuz i was drawing like crazy with hourglasses..and had 2 time quanta left just b4 i decked lolz O_o and also i went to go put up an oty with a ulitharid on oponent side..thinking i had a quint buut i didnt hah

[icybraker]

Adding to the list:

"52 cards is kinda an ugly number; I added 8 extra pillars to my deck to get a nice, round 60."
"Whoa, fire dragons are REALLY awesome! I'm going to save up enough money to buy 20 of them and then put them in my deck!"
"Yeah, I like to click whatever I can, whenever I can. Even if there are no monsters on the field, I play 3 Sundials just in case. Oh look, I got Quintessence! Let's make my Skeleton immortal haha."

On a similar note:

"Uh oh! My Otyugh has Gravity Pull on it! Let's immortalize it!"
"Uh oh! My dragon is poisoned! Let's immortalize it!"
"Uh oh! My Spark will die next turn! Let's immortalize it!"

[Wisemage]

i once put  a quint on a poisoned oty, total face palm moment.

[hermantheowl]

Clicking on an unpgraded shard to see the <<WARNING YOU ARE ABOUT TO SELL A RARE>> message.

[PirateKingAtomsk]

Voting has started

[icybraker]

The first 3 options aren't really huge errors, you know.

[pikachufan2164]

"OMG -- there's a hacker on PvP!!!11111oneoneonetwothree!"

It's called de-sync, kiddies. 

[Wisemage]

i went with the first 2.

More than once i have switched to my pvp deck without changing marks, it screws me over.

Also more than once ive had an oty and a quint in my hand, i play the oty then realize i cant paly the quint cause no aether mana.

[PhuzzY LogiK]

"OMG -- there's a hacker on PvP!!!11111oneoneonetwothree!"

It's called de-sync, kiddies. 

In all fairness, most of the time when you're new to a multiplayer online game and you see something really screwy, it's not outlandish to think someone is doing something they shouldn't be.