What causes a standard transmission to select two gears at the same time?

A standard transmission selecting two gears simultaneously is typically a result of worn shift rail interlocks. These interlocks are designed to prevent multiple gears from being engaged at the same time, ensuring smooth gear shifts and proper functioning of the transmission. When these interlocks become worn, they can fail to adequately prevent this scenario, leading to both gears being selected, which can cause severe damage to the transmission.

Other factors, while they may cause issues in the transmission's overall operation, do not specifically lead to the simultaneous engagement of two gears. For instance, worn main drive gear teeth would primarily affect the ability of the gears to mesh properly but would not inherently cause two gears to engage at once. Excessive flywheel run-out could lead to other problems such as clutch engagement issues, but it wouldn't result in the selection of two gears. Faulty gearbox seals mainly involve leakage concerns and do not affect the gear engagement mechanism directly. Thus, worn shift rail interlocks are the primary factor responsible for this malfunction.