The NFL's "Magic Yellow Line" was developed by a company called Sportvision in the late 1990's and took the football world by storm.  The line is actually much more complicated to implement during a game that you might think.  The line costs tens of thousands of dollars each broadcast that it is used for.  The video below explains the ins and outs of the magic yellow line and it actually is quite interesting.  Plus, the video shows an example of the type of conditions it takes to make the line not work how it is intended to.