It's a tiny bug, I know, but it's been bugging me for a while.
On your Microsoft Account login screen there is the form for email address and password, a checkbox to check which disallows your login to time out, and a button to submit the form.
All is well except that when checking the checkbox, both the check and the background of the page behind it are pure black, so it is not visually possible to determine whether the box has been successfully checked or not.
Black on black just isn't very perceptible.
Again, there's no functional loss - only a visual ambiguity.
A user might click on the checkbox once and assume that they must have missed it when they clicked or tapped and try clicking it again (thus unchecking it) and might then be uncertain what state the checkbox was in without reloading the form.