When I play my Death Knight, Strangulate gets this honor. In PvE, I may come up against a pack of annoying spell casters, but for some reason, it never occurs to me reach out a choke a witch. It'd get them rounded up easier and increase my chances of survival, but I'm so busy keeping up my rotation I forget to fit it in. Of course, in PvP, I'm expecting to do a lot of spell interrupting, so I remember it then, which makes my blind spot in PvE all the more silly.
On my Warlock, I keep forgetting that I could avoid a lot more corpse runs while soloing if I used my Soul Stone on myself and kept a Health Stone summoned. Sure, In theory I might get a group at any moment and then have to deal with my stone being on cool down, but honestly, I'm a DPS Warlock, they're a dime a dozen, and the chance of me getting a group when I'm not actively look for it are probably slim. So there's not really an excuse not to self-soulstone. But then I forget and I curse myself as I run through the Fjords.
I'm curious to know if I'm the only one. Do you have a minor skill that you keep forgetting to use? Did you overcome skill forgetfulness? If so, how? Was it a mimetic? An addon? Pure stubbornness?