Nintendo happily granted a license to Tengen, eager to add Atari Games' arcade hits to the NES library. However Tengen was apparently less than enthusiastic about the terms of the license, and secretly worked to defeat the Nintendo lock-out chip. While numerous manufacturers managed to do over-ride this chip by zapping it with a voltage spike, Tengen engineers feared this could potentially damage NES consoles and expose them to unnecessary liability. Instead they chose to reverse engineer the chip and decipher the code required to unlock it. Unfortunately the engineers were unable to do so, and launch date for their first batch of games was rapidly approaching.
In desperation, Tengen turned to the US Copyright Office. Their lawyers contacted the government office to request a copy of the Nintendo lock-out program, claiming they needed it for potential litigation against Nintendo. Once obtained, they used the program to create their own chip that would unlock the NES. When Tengen launched the unlicensed versions of their games, Nintendo immediately sued Tengen for breach of contract. Eventually Tengen was forced to admit their duplicity and pay damages to Nintendo.
Tengen's unlicensed NES game cartridges do not come in the universally recognizable semi-square grey shape regular Nintendo licensed games come in, but instead are rounded and matte-black, more resembling the original Atari catridges. As a side note, for Tengen's involvement in the complex story of Tetris on the NES, click on this Tetris link.