An exploit on Microsoft's download page allows users to download a free copy of Windows 8 Pro for free.

Microsoft is offering free upgrades for the Windows 8 Media Center add-on. However, once you are download the update, you are free to go with a valid version of Windows 8 Pro + Windows Media Center OS, since Microsoft does not check the validity of the original key used to activate the copy of Windows 8 Pro before dishing out another key.

Here is how the situation is described on Reddit, by a user:

"...But, Microsoft is giving away free upgrade keys to turn Windows 8 Pro into Windows 8 Pro+WMC [Windows Media Center, which is a separate add-on for Windows 8], and they don't bother to check if the key you are currently using is valid. As long as the system is activated, even if it is activated with KMS [Key Management Service], it will take the key and apply the upgrade. Since the WMC key comes directly from Microsoft and is unique for each user, it activates Windows 8 Pro permanently..."

Microsoft has not commented on the report yet.