This occurs when someone on your buddy list, who used to be a friend, was never deleted from the list. You mean to click the video chat icon next to your friend's name, but accidentally click the used-to-be friend's name. You immediately cancel the invitation and block the person, to prevent the incident from re-occurring.
Alexa: Shit! I just accidentally invited Kyle to a video chat and then canceled it immediately. Now I have to block him so that never happens again!
Jacob: Haha I just got the accidental videochat block from John. I havent talked to him in months and he invited me to a video chat, canceled a second later, and then blocked me.