-- WRONG:

net.Receieve("oiutfit", function()
    local ply = net.ReadPlayer()
    local data = net.ReadData()

    if ply:IsValid() then
        ply:AttachData(data)
    end
end)

-- CORRECT:

local queue

net.Receieve("oiutfit", function()
    local ply = net.ReadPlayer()
    local data = net.ReadData()

    table.insert(queue, {ply = ply, data = data}
end)

hook.Add("Think", "queueprocess", function()
    for k,v  in pairs(queue) do
        if v.ply:IsValid() then
            ply:AttachData(v.data)
            queue[k] = nil
        end
    end
end)