Final answer:
A twisted mainshaft in a standard transmission is most likely caused by shock loads or not using the clutch when shifting, which introduce significant stress to the mainshaft.
Step-by-step explanation:
Factors that could potentially lead to a twisted mainshaft in a standard transmission include shock loads encountered during vehicle operation, such as rapid acceleration or deceleration, and hitting potholes or road debris. In addition, not using the clutch when shifting can also overstress the mainshaft, leading to twisting or bending. Both improper driveline operating angles and excessive mainshaft gear tolerances can lead to abnormal stresses on the mainshaft; however, they are less likely to cause a twisted mainshaft compared to the direct, immediate impacts of shock loads and improper shifting without the clutch.