I admit, this is a total click bait title for the post.
I am sure there are multiple mistakes you should avoid with your will–this is why you should absolutely work with a lawyer on this sort of thing.
(Perhaps the biggest mistake you might make is having no will at all, but that’s a different story).
Anyway, I was prompted to write this because my very close friend died, and her family could not find her will.
No one knew who her lawyer was, no one could find it in her home.
Her mother knew she was to be the executor (or executrix) of the will, but without a will to brandish, she couldn’t get anything done. Including getting into my friend’s safety deposit box to find said will.
Luckily, my friend had arranged for me to have access to her safety deposit box 15 years ago when she first got it. I was allowed to open it last week–in the presence of multiple bank employees–to find and retrieve her will (and only her will). Thank goodness the will was actually there.
If not, there is actually a process in place to get the county to provide a copy of a person’s will, though it’s not clear how long it takes to receive it. As her family is from out of town, they probably would not have been able to obtain the will and get business started before they leave again, if they had to go this route.

So, please, if you are going to be responsible and make a will: Please make sure a (valid) copy is available to those who will need it. It should not be in a safety deposit box, where legal issues may keep it isolated for too long, or hidden so well no one can find it.

This is very good advice. You’ve reminded me that I need to update my emergency binder.
I’ve mostly been thinking about updating my will (since several back up executors are no longer with us), but keeping my emergency info available (but not so available that strangers in the house aren’t going to find it) is also on my mind now.