Counter-Invader version 10.14 and later contains a bug in the code for the portal that let you proceed to the next level. In level 12 and some later level, a bug will cause you to get stuck. When that occurs, press F12 to force a jump to the next level and continue playing.

The F12 short key is a debug feature added for myself and my official beta testers to assist in testing and bug tracking. Feel free to use and abuse as you'd like. Consider it a cheat code.

NOTE: You will miss out on lots of bonuses if you skip levels without completing them.