Today I set up a download directory for one of our customers. I turned on basic authentication, gave a nice /=%27gujdw765 password and tried out from Firefox.
Everything worked. Then I tried via IE11. IE started to prompt me for credentials again and again.

I checked error log on webserver and found password mismatch messages. Tried again with same results.
Tried on my second machine with same results again.

I sent over the info to my deskmate. He tried the same in IE and it worked for him!
What the hell is happening to my machines?

I modified apache config to log Authorization header value:

CustomLog ${APACHE_LOG_DIR}/access.log  "\"%{Authorization}i\""

Now from access log it becomes clear, that my IE does not send my password over the wire!
E.g. in case of typed user name “myuser” and password “blahblah”:

"Basic bXl1c2VyOgo="

I started some experiments and found that sometimes the password was sent sometimes not.
But why?

You will never guess.

Because I was copy&pasing that nice password via clipboard AND used Shift+Insert key combination for pasting!
When I was doing my experiments I was typing directly “12345” into password field and it was sent gracefully.
My deskmate was using Ctrl+V which was working too.
But no one was able to send the password pasted via Shift+Insert!

After these it was easy to find the official info: https://support.microsoft.com/en-us/kb/2547752
It is known problem since 2011 and there is a fix for it.
But why wasnt it distributed via Windows Update since then?