For me, it's the price stickers on graded card sleeves! I MUST take off the sticker and ensure there is no sticker residue (this is done immediately after purchase or when I receive the card in the mail)! If there is, it's in the garbage and replaced with a fresh clean sleeve! This is for cards I'm holding in my PC or selling/trading.
Another thing is going through dollar boxes, I MUST go through the cards and place them face up and continue going through a stack. No upside down, wrong side, etc. cards!
What are your hobby related quirky-things?