I was doing a HH brisket today and on an early check, the Weber lid temp was off scale high. Pointed to about 6:30 or 7 o'clock. I stuck my thermopen into the lid vent and it registered 410.
Needless to say I closed the door, which had been propped open. Closed the vents to 50%, including the lid. After the temps came down, I reopened all to 100 percent but kept door closed for remaining cook. Lid stabilized at 335.