Flash lag effect
There are many reasons why we, as assistant referees, get an offside decision wrong. Sometimes, it is because of we are not focused. Some other times, we are not properly aligned with the offside line. But there are times when we get our call wrong even when we are properly aligned and remain focused. One of the reasons for this is the flash lag effect.
Your eyes trick your brain. Sometimes when two objects are not moving in the same direction, there is a delay between what our eyes see and what our brain registers. It causes a certain object to appear in our brain that it is in a different spot than it actually is. There are numerous websites that explain this effect so if you are unfamiliar with it, we recommend you take a look.
Here is an example of an assistant referee incorrectly calling an onside player as offside, even though he was properly aligned with the second-last defender.