Delayed or held attacks can sometimes have the illusion of going through your blocks, but that is more player error than problem with the game. - already mentioned
also turning too much into or away from your blocks - already mentioned
Another problem is packet loss, which can cause YOU on your side to be shown with the correct block, lets say an overhead, but your opponent sees you holding a down block, and continues through with his over head attack which on your screen looks like it should have been blocked but wasn't.
Happens a bit on duels, where there is actually time to talk to one another to see wtf went wrong why your opponent was sitting there so confidently with a down block ready to receive your overhead.
Another form of this is blocking with the complete wrong attack, IE, blocking side swings with over heads or down blocks, animation is showing the wrong sequence but it was the correct block.
info gathered through experience, im not a computer tech or anything so I may be totally wrong as to what to call it but this is what happens for me and a few others I've talked to about it.