Sometimes when you step off a pressure plate that is connected to something, for example a fence gate, the gate stays open for a longer period of time then it should. Same thing vise versa so when u stand on the pressure plate, the fence gate is closed, but the gate isn't really closed. You can see a transparent version of the fence gate open when it looks closed. Sadly, I couldn't take a screenshot quick enough too see that image, but here's what I got