Please see the below image:
As you can see, I am voting on my own question having 3 upvotes. The question's score displayed as 4 instead of 3 for two seconds and then displayed a box saying "An error has occured - please retry your request" instead of not allowing the user to upvote on his question by displaying "You can't vote your own post". Why is this happening? I verified in other SE sites, those are working fine.