I have been having this issue with my computer running Windows 8.1. I haven't noticed it in other versions of Windows, but I'm not certain that it is specific to this version.
When I type my password at the lock screen, and then hit enter, it often passes the keypress from the enter key through to whichever window is active as soon as the screen unlocks. If I am very careful, I can tap the enter key very quickly, and it will not pass through, but if I just type normally, it seems like the enter key is pressed for long enough that the active window also receives that keypress. This is normally a slight annoyance, as it will open a conversation in my Gmail or something of the sort, but occasionally I have a program running that requires interaction, and if I didn't want the default, I'm out of luck.
It seems that the issue may have started when I got a new Razer mechanical keyboard. My old non-mechanical keyboard does not have the same behavior. Is there something different about how they operate that would cause a single keypress to be interpreted at multiple keypresses? I don't see anything in the software, or elsewhere online that would indicate any setting that would cause this in Razer's software.
Any ideas what else might be causing this, and if there is a setting I could change somewhere to avoid this, rather than just being really careful about how I hit the enter key when unlocking the screen?