Star Trek: Enterprise
Unfortunately, this single production mistake has caused much confusion within the fan community, as well as another production mistake decades later. There is a fan theory that each ship has its own unique insignia and that Starfleet only universally adopted the delta symbol in honour of the Enterprise returning from its very successful five year mission. This fan theory also led to a production mistake in the Star Trek: Enterprise episode “In a Mirror Darkly” where we see what happens to the lost USS "Defiant" that first appeared in the original series episode “The Tholian Web”. Even though in “The Tholian Web” we see the crew of the Defiant wearing the delta insignia the Star Trek: Enterprise costume designers created a unique insignia for the "Defiant" uniform.