I'll give as detailed a description as possible, but providing a video would take me a while.
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This was something I encountered on the original VBA, but I wonder if maybe it will happen on VBA-M as well (I haven't played far enough using VBA-M to check).
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Anyway. In the Battle Network games, the are many virus enemies running around. Some of them launch projectiles in parabolic arcs to their target: You. I've confirmed this happens in BN2, 3, 5, and 6 so far.
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Basically: All the enemies who launch in-game objects seem to have some kind of error or bug that causes weird things to happen.
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For the Beetank / Bugtanks in BN2 and 5 (They behave in exactly the same way) The Beetanks will move up and down their column, and play the firing animation as normal. However. Sometimes the Viruses will seem to misfire and not fire any projectile at all. Making them sometimes unable to attack in a pseudorandom manner.
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The Beetle Viruses of BN3 have a similar problem, except they behave differently, by moving keeping a distances of two panels between you and it, and staying off the row you're on. Sometimes when they fire, nothing happens.
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The Killerplant Viruses in BN2 add more specifics to this problem. If a Killerplant family virus is on the top row in the enemy area, unless you happen to be on the same row, they will be unable to launch their TreeBombs at all. This has shown to be the case as well if you are on the top row while the Killerplant is on the bottom one.
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Then so far, with BN6. The Puffy Virus launches a trio of needles at you into your area...The problem is, the projectiles are completely invisible, despite the attack succeeding to execute.
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A notable symptom of this inherent error is that for some of the virues, the projectile can sometimes be seen somewhere on the screen (usually near the top, or very bottom of the emulator screen) for about 1-2 frames before disappearing without a trace.
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I have the original BN6 for Gregar, and I have owned at least one of every game in the series at some point in time, and on top of that, I have an acute long-term memory, and I don't remember any of the things mentioned above ever happening on the original carts.
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Incidently, none of the BattleChip attacks in the game that launch objects in an arc have this kind of problem at all.
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Due to my current understanding of programming (not all that good, but I can do some basic C++), I have come to the conclusion the cause is either the emulator (how it is handling on-screen objects and stuff) or the ROM itself, and I simply find it hard to believe that a series of ROMs could possibly experience the same exact kind of problem across all six games.