Yup, I've been following along and I also finally got the bugged one by: reopening Change Wallpaper after having closed it with a custom color being selected; and then clicking "custom color".
In a way it's a perfect little example of how a bug can seem obscure to some (most) users/developers but seem glaring and unacceptable to a few (the few who happen to use the relevant feature a lot).
If some engineer at Apple ever gets to fixing this, that would make an exceptional story! Really interesting WHY these circumstances exactly lead to the bugged popup, I can't even imagine what's going wrong there.
In a way it's a perfect little example of how a bug can seem obscure to some (most) users/developers but seem glaring and unacceptable to a few (the few who happen to use the relevant feature a lot).