Forgotten abilities and plot holes

This is an eternal problem for readers. Writers don't really tend to care about it too much, but readers HATE it!
What am I talking about? Basically, it's when a character gains a power, acquires an item, learns some info or a spell, meets a magical person or whatever: they gain something that makes them very powerful or is exceptionally useful and it helps them get out of a jam but then they never use it again! No matter how easily they could use it to get out of a tricky situation or how easily they could beat the villain with it, they've totally forgotten that they have it. This is especially an issue in sequels because writers like to pretend their characters start with a blank slate.

As a reader (or audience) you become invested in the character and the world they exist in, so this sort of ability forgetting is especially galling. You mentally scream at the character "USE THE THING YOU IDIOT!", but to no avail because the writer forgot about it and so it causes a pothole.

Why does this happen?
99% of the time it's bad writing. Not extremely bad writing, it's just the normal, every day bad writing that we're all prone to. It's like obvious spelling or grammar mistakes: you check your work over and over and see nothing wrong, you're completely blind to it. That's exactly what happens with these plot-holes, as a creator they're invisible to you, but to the reader they stand out like a bloodstain. They're so easy to miss so they rarely get fixed and if the creator does find them it's usually too late anyway because it would take too much work to change things.

But why do these things cause plot holes in the first place?
The answer is even worse writing. The creator needs a McGuffin to get out of a problem, or an easy climax because audiences LOVE characters to gain small victories, so they introduce a thing without thinking ahead about what that could "really" mean for the story or all the different ways the character could use it. The creator is lazy and flying by the seat of their pants… like many of us webcomicers are prone to, and this causes big issues down the road.

What are some ways to get around it?
This happens a lot any time you have an overpowered character who gets even more powerful. In Doctor Who it was the "Sonic Screwdriver". The device was a lazy McGuffin but gained even more powers as the series went on and the writers got lazier. They solved it by simply writing it out in the end- though they continued to reintroduce it later for fans. The trouble with McGuffin's like that is once you have them they can be very hard to get rid off. In superhero comics Superman is a great example with all his many powers and magically massive strength. The way they fix that is through a number of strategies: having him lose powers, having enemies nullify them, wiping out timelines and just forgetting he has them. Goku of the Dragon Ball franchise is always gaining strength and abilities because that's the format of the series. The way they get around it is by always matching him with enemies of greater power… which is a very flawed solution for too many reasons to mention.

Another method is for the writer to belatedly acknowledge the forgotten ability and make a joke out of it, "Oh, if only I'd used that thing I found back in the tombs, that could have saved all this trouble. Too bad I forgot about it till now, DOH!". Others will retconn in reasons, "It was lost". This is bad and readers rarely buy it.

The best approach is to think ahead
Don't just throw in powers, abilities, useful connections and devices willy-nilly, think ahead. Not just in terms of your current plot but future stories, think about how it'll change things for the character, what will it take to nullify in order to make your story interesting again etc. In some cases you'll find that generates great ideas for new storylines and plot-points, in others you'll realise it's just too much trouble to bother with, but as long as you've thought ahead you can easily and simply make the character lose the thing after it's done its job and therefore totally avoid a plot hole. :)

This won't help you avoid all plot holes
I'm only talking about those caused by forgotten powers and abilities. 😉

Why don't the writers (usually) care?
Because we're lazy which is why we used those tricks to begin with, and it's too much trouble to fix the issues most of the time so we just pretend they don't exist.