An aveira b’shogeg or b’oneis can still happen (not necessarily piggul), i. e. a person would probably still be able to accidentally hit a light switch on shabbos.
Shogeg is defined by transgressing a prohibition without knowledge – either you didn’t know it was forbidden, or you didn’t know that the circumstances involved a prohibition. Eg, you thought it was permitted to switch on a light on Shabbos, or that it wasn’t Shabbos.
Accidentally hitting a light switch isn’t a שוגג, it is מתעסק. You are unaware of the action you are doing. The Achronim debate the exact halachic status of מתעסק, but many consider it (at least on Shabbos, where you need מלאכת מחשבת) not to be a transgression at all. According to plenty of Poskim, if you see someone leaning against the wall about to hit a light switch unawares, you do not need to stop him.