Other common causes are worn gaskets, faulty seals, or even a loose oil filter. Since the engine holds several quarts of oil, it won't miss small drops. But a sizable stain requires immediate attention. If the oil appears very dark even though you change it frequently, combustion products may be working into the crankcase through worn piston rings or a damaged cylinder. If the oil has a frothy or milky appearance, it may be contaminated with coolant, probably via a blown head gasket or a cracked engine block.
Metal shavings in the residue indicate significant internal damage.
Common causes of more serious transmission leaks are loose bolts or a worn-out gasket on the oil pan or a bad front or rear oil seal. The color and odor of the fluid tells you a lot about the condition of the transmission, so pay attention. As long as the fluid smells oily and looks good, everything is fine. If the fluid again turns dark, the transmission probably needs to be rebuilt. If the fluid has turned milky, it probably has been contaminated by coolant, commonly caused by a leak in the transmission cooler located in the radiator.