Mux, a startup focused on video, shared their experience creating an advertisement for BART, San Francisco's underground transit system. They discovered that BART does not allow any code in ads and faced challenges when their ad featuring a terminal with blinking cursor was rejected. The company went through the process of redesigning the ad but ultimately had to put it on hold due to time constraints. Despite this setback, Mux used the final video for their homepage and plans to continue pursuing out-of-home advertising opportunities.