I have the issue with a MacBook Pro 15" Mid-2014, using the latest OS, and have intermittently seen the issue for several months. Most of the time, it's not an issue. My issue is the 'n' key. It works fine for weeks. Then it stops completely and I have to find some other 'n' and "N" to copy and paste for that often-used character. After a few minutes, a bunch of nnnnnnnn's show up, almost like it was buffering them up for the last several minutes. It doesn't dump them all in at once, but I just had the issue a moment ago. When it transitions to working and I press an 'n' key, a tooltip pops up showing ñ and some other n I think, which I only get when this issue is about to go away.
Then, if I wrote this sentence, when it had a problem, and then again later, it might turn out like this...
The, if I wrote this setece, whe it had a problem, ad the agai later, it might tur out like this...
Thnen, nnif I wronte thisn sentence, when it had a problem, annd then again later, nnnnit might tnurn out linke this...
I sometimes wonder if dirt got into the tiny gaps of the keyboard, which I wish were sealed. But when the problem resolves itself, and letters come up with no key motion, it does make me think the OS/firmware might be the issue, although a mechanical issue such as dirt isn't completely ruled out. Again, sealed keys would help prevent many key issues, and it would make them easier to clean.
