The Z80 had them. Even the HP41C calculator had some undocumented codes. The HCF (Halt and Catch Fire) instruction was apocryphal, but we always heard the old video controller chips could be ...

From 2014 to 2017, a TV series titled Halt and Catch Fire aired on the AMC cable channel. Comprising 40 episodes, it was also available at various times on Netflix and other streaming channels.