One problem related to transmission fluid leaking has been reported for the 2013 Ford Mustang. The most recently reported issues are listed below. Please also check out the statistics and reliability analysis of the 2013 Ford Mustang based on all problems reported for the 2013 Mustang.
Approximately 150 miles into a trip while traveling on interstate in 6th gear at 80 mph I had to slow for break lights ahead. Tried to downshift into 5th and immediately the tach red lined and the transmission would not engage. Tried to go back to 6th and same issue. Used the foot brake to slow to a stop and was able to drive in 1st through 4th gear or reverse. Drove to nearest Ford dealership in 4th gear for repair. The external diagnosis was fine so the tech went to remove the transmission. He noticed the vent cap missing off the transmission and only 1 qt of fluid left. There was a burnt smell and metal shavings in the assembly. The transmission was replaced due to catastrophic failure. Just 3 days prior to this the car had been serviced and the transmission, cap, fluid all checked out well. The tech at the dealership explained the pressure inside the transmission could cause a major burn-off of the fluid if the cap came off as it appeared. So a slow leak was not the case here. The removal of the vent cap was the root cause.