Bugs and Glitches are errors in programming or faults in which unexpected events arise. As shown above, the game is suffering from a glitch, in that the enemy (red) is flying off the screen, under the floor. This may relate to a bug in collision, or paths. Bugs and Glitches often negatively impact gameplay and reviews, and can prove irritating. Whilst they can be beneficial, this could be considered exploitation, which may spoil the game.
The best way is to run your game in a debug mode and search for the bug, identify the cause, then work on fixing it. A basic guide on bug fixing is available at Bug Fixing.