I've noticed that this started happening maybe a few weeks ago:
When I'm logging in, sometimes when I type an underscore in my password, it does not register as a keypress, i.e. no black dot appears in the password field. It only happens some of the time.
I noticed it because a few times when I was logging in, it said I typed the password incorrectly (because the underscore was missing). I then began to type the password slowly upon logging in after those few times, and I noticed that the underscore would fail to be recognized at times.
I'm quite sure I'm typing it properly, because now I will very slowly and deliberately type the password with the intent of watching this bug happen. Typically it will happen, and then I will type the underscore again and it will register.
This happens both on my MBP's keyboard and with a USB keyboard. It's pretty easy to replicate, as it will happen almost every time I have to unlock the screen from sleep.