When the characters have one very convenient ability that would be useful in a lot of scenes… and the writer forgets the character can do that more times than not.

  
Goku’s mind reading (Dragon ball Z): Goku reads Krillin’s mind to catch up to what had happened while he was getting to Namek, this technique isn’t explained nor used again.

  
Humungosaur‘s growing (Ben 10): all the UA aliens had lots of abilities that stacked in top of eachother form time to time. However Ben’s most used alien (89 transformations) had only two things, super strength and the ability to change size… that ability got forgotten in Ben 10 Omniverse since he never grows throughout the show.

  
Weiss’s Time dilation (RWBY): Weiss can alter the speed of others, using it on herself and Blake during the second volume (Blake to react to missiles before they get close, and Weiss to speed blitz a lieutenant… that still beats her).

despite this being arguably one of her strongest abilities (next to summoning monsters, and using dust for elemental infused attacks) it’s completely forgotten in later seasons and never brought up again.

  
Shadow’s air shoes (Sonic adventure 2): Shadow is shown at the beginning of Sonic adventure 2 with his air shoes (yes, that’s what they are) allowing him to fly… this is repeatedly forgotten by Sega.

He needs rouge’s help to fly in Sonic heroes, and the doom wings to fly in Shadow generations.

  
luckily this is mostly a games thing since Sonic 3 the movie, Sonic prime and other mediums do show shadow using the flying abilities of his shoes.

[HATED trope] the writer forgets the character’s powers