∑ If you do not miss any shots your bonus score will keep on doubling, up to 10,000, and then reset to 000. If you miss a shot when itís at 10,000, the bonus score will revert to 5,000.
∑ BUG: In the challenge stage, if you shoot all 40 aliens you normally get a bonus charge of 60. Sometimes a red hydra comes out during this stage. If you shoot it and all 40 aliens, you get a total of 41. The game seems to fail the test of 'hits=40' and only gives you a bonus charge of 41.