Hacker15e
Who am I? Where are my pants?
Look at that wrinkled skin.
The B models are the most abused of all the T-38 airframes. Due to the nature of the course they were used to teach (Lead In Fighter Training, later renamed Introduction to Fighter Fundamentals), they were exposed to a lot more G and abrupt maneuvering than their A model counterparts. The LIFT and IFF courses are a USAF pilot's first real exposure to aggressive maneuvering, sustained high-G maneuvering, and very abrupt flight control inputs (for example, abrupt, rapid full deflection of the stick all the way aft to the seat-pan at 330 knots!). A lot of ham-fisted student fighter pilots have over-G'd the crap out of those jets many times.
That being said, the wrinkles in the boat-tail (that area right above the horizontal stab and back to the burner cans) are pretty standard T-38 fare. The horizontal attaches to the boat-tail, which also holds the "ejector nozzles". That area is a main structural piece, and gets a TON of stress.
Here you can see where the boat-tail removes from the rest of the airframe for access to the engines.
This is looking straight up into the boat-tail, and you can see that there is a lot of stress on there with the horizontal stab (although they call it the "slab" because it is a "flying tail" with both sides moving together) being mounted to this piece.