since we're talking cars here
In racing, T-boning often occurs when a driver loses it "in the marbles" but, as often as not, the base of the "T" is a wall, not another vehicle.

"In the marbles" is the area of the track near the top of the high oval where bits of distressed tires, having the appearance of marbles, tend to accumulate to the peril of drivers entering this zone. As you might expect, it is hard to maintain control when you are skidding on rubber beads.
[Come to think of it, truck drivers worry about ending up "in the rhubarb" when they fall asleep at the wheel ... but that is certainly better than being T-boned.]

When we lose control of our mental faculties, people say we have "lost our marbles". Obviously, this has nothing to do with car racing. Where does this expression come from - losing one's marbles - I wonder?