Hearthpwn

Quote from Dinomaniak >>

Believe it or not, there was a day when I actually took the game seriously.
Nobody knows the amount of brain cells I've lost due to anger that very day.

 You're not wrong - there is something inherently strange in just how much of a red mist Hearthstone can seem to make descend upon you when you lose a game in seemingly huge unfariness of circumstances. 
I've never known another game (or activity) like it to have a similar effect (on me, at least).

In fact on that note, I'll add a 4 to my earlier list.
4. Allowing Hearthstone to rule my emotions when I've lost a game badly and then been in a foul mood to those around me directly afterwards. They certainly didn't deserve that! ;-(

1. Dusting a golden Hadronax about 2 weeks before Hadronax Druid appeared on the scene and started wiping the floor with it.

2. Week 1 of one of the early expansions crafting about 5 different legendaries that turned out to virtually never see play in any competitive decks.

3. Thinking that hitting Legendary ranks was going to be this bright and amazing "thing" only to discover that it's little more than "more of the same".