Question to who know programing (position)

[ZohnoReecho]

If you bring position check client side you could seriously cheat.
The client would have to send to the server "all ok, he is safe" when you avoid an hit. You could seriously cheat forcing the client to send that message everytime, no matter where you are.

A previous bug kinda worked that way. You told the server what you wanted to sell and it did! Without even checking if you had that item.
FFXI manages stuff like this differently. The client tells the server which inventory slot you want to sell/equip, the server checks which item you have in that slot (so you don't send the item id from the client, but the position) and then it's sold/equipped.

In the end, you should avoid any cheatable check on the client side. Even the teleporting issue should have a server side check, as for example "if (point B - point A) > (allowed distance in timeframe) then banhammer!". Probably this would increase the server load because the info required to do this would double (old pos and new pos instead of just new pos) and there is math to do, but I think it would help with fighting cheaters.