As ARs, we sometimes make over 20 onside/offside decisions per match. In order to make a correct decision, not only do we have to be aligned with the offside line but also we must be fully focused. When our focus level goes down even for a quarter of a second, it can lead to a mistake.
Furthermore, some offside decisions are match critical. There are times when our onside decision allows an obvious goal scoring opportunity to happen, or our offside decision denies an OGSO. When a player denies an OGSO, they are sent off (or cautions in some instances) because for soccer, such an offence is extremely serious. This is to say that when an assistant referee makes a mistake and allows an OGSO to happen or denied an OGSO, it is a major mistake.
Watch the video below. The attacker was onside by a yard. The attacker continues to take a shot on the goal in a 1 v 1 situation with the goal keeper. The AR flag, however, goes up erroneously. Stay focused and avoid these critical mistakes.