Percy x Annabeth in the Percy Jackson/Camp Half-Blood series. Annabeth is an abusive brat. Your response to someone not picking up on your hints that you want to dance is not punching them in the gut. You don't judo-flip someone you "love" for something that wasn't even his fault (getting kidnapped in this case). You know the author would have never done this if the genders were reversed.

Levi x Petra from Attack on Titan. I'm not saying this applies to all shippers but it seems like 8/10 times these shippers will moan about shipping Levi with Eren or Mikasa. Levi, to my knowledge, is never given an exact age, just that he's in his 30s, I've seen a lot of the fandom settle for 34 for the bulk of the series. Eren and Mikasa are 15 for the bulk of the series. The issue is Petra's implied to be barely older than Eren. Her father said she was too young to marry, she wasn't let in on Erwin's plan to weed out the traitor in the Survey Corps because she joined within the last five years. Soldiers begin training at twelve and finish by fifteen. She's not much older than Eren, she's too young for Levi, too. It's the blatant hypocrisy that drives me nuts. (Especially since Hange and Erwin are right there!)