Win 8.1 .
I have two screens of different resolutions. During the first attempts, I only played on one screen. On the second or third attempt, I plugged in two screens and the window started to look strange. It was in the window, but without a frame and not on the full screen. It seems that Alt+Enter did not work. I have expanded to full screen via options. I started playing and then the character started twitching and sometimes passing through the gate. Also, it may have started after jumping off and climbing stairs many times...
Remembered. Also, after the first pass, I moved the game folder from one disk to another.
I wouldn't be surprised that this is a problem with my PC - it's old and the motherboard was under repair,