Transmission in Safe (Limp) mode is when the transmission only works in third gear only, all the gear selector indicators all light up. I still don’t know what causes this, but if you drive the car home (in third gear only ~ 40-45 MPH max) or leave it for about 2 hours, it will reset itself. Before towing or driving it to the shop, let the car rest for a few hours. For all the years I’ve owned Audi, it happened to me once today on my 96 A6 Avant. Drove the car locally home, left it on the drive way and went to dinner at 6:30PM, got back at 8:45PM, came out to start the car, everything is fine.
This also happened to my 89 BMW 735iL many moons ago.