This page details bugs of Silkworm (NES).
After beating True Last Boss (Wave 9), there is a short animation of Heli landing and Jeep moving to the leftmost part of the screen followed by the cracking sounds on the black screen and then the final cutscene.
Before this, the game need to check whether the Jeep or Heli still alive. If one of the players isn't get to this point a corresponding animated object does not display. This works fine if both players or just Heli still alive.
If Heli is dead and only Jeep left, the program additionally perform a special test at $CB93 to disable the Heli landing sound. But, branch at this location has a wrong offset ($04 instead of $02) so jump goes to the bad opcode and corrupt current routine.
However, this at this particular point this not cause the whole program corruption. Instead game clears the screen and start the cutscene immediately with the cracking sound on the black screen as mentioned above. No one noticed.
To fix this bug and test how this should actually work if only Jeep is alive you may use Game Genie code ZEOGKLGA.