The Fish Men of Konami's beloved Castlevania have been frustrating and irritating players since the franchise's debut in 1986.  One minute you're walking along a rickety platform above one of those bottomless pits of water that Dracula's civil engineer designed in his castle, the next you're knocked backward into the abyss by the arrival of a Fish Man leaping up from the depths.  How do they always seem to know where you are?  Can they be avoided?  Do they have a hidden tell you could use to determine where one may next appear?  As it turns out, the Fish Men completely give away their routine if you know where to look.  Chances are that you've seen it for thirty years now and just never noticed.  JHarris at MetaFilter, as part of an amazing comment on the behavior of some of the original Castlevania's enemies such as Medusa Heads and Bats, explains how you too can learn to safely dodge the Fish Men.

Example 2: The fishmen in Stage 10.

The first stage of this block is the flooded passage level, the one where you have to jump between precarious moving platforms, dealing with ill-timed bats, and sometimes fishmen jumping out terrifyingly close to you.

Like medusa heads, they always come out on a timer, and there is actually nothing random about their behavior. And the first loop through the game, fishmen are actually somewhat courteous, and will never leap into the air directly beneath your location. You still have to deal with their descent, though, and the bats attacking, and fireballs spat by fishmen who have landed on solid ground. Stage 10 is a notorious sticking place for many players.

When you beat the game and come back to Block 4 on the second loop, the difficulty is jacked up quite a bit. Actually, the increased difficulty really only holds for the first four blocks; Blocks 5 and 6 are nearly identical to the first loop. So, Block 4 is really the game's last stand. If you can consistently beat it and everything up to it, you have defeated Castlevania.

Perhaps because of this, the fishmen in Block 4 no longer kindly avoid leaping out of the water if you're directly over them. The only way to avoid dying to them is knowing where they'll leap from, and just not being over that spot at the moment they spawn.

Unfair, right? Having to memorize an obscure series of locations and constantly making sure you're not over one when the fatal leap occurs, right? How is a player supposed to know that?

Well it turns out, there is a tell to the fishmen's spawn locations in this stage. When I first noticed it, I thought to myself, how diabolical. Once you realize it, you'll never forget their spawn locations again. It works in the first loop too, but it's less essential. Here it is. This is a spoiler, but you probably won't notice yourself your first dozen games anyway:

With two exceptions, fishmen can only spawn directly beneath candle locations.

If you whip a candle, fishmen can still spawn there, although if you're wise you've already on the move forward. This is a tremendous aid to getting through Castlevania's most chaotic level. The two exceptions are both at the moving platforms with the low-overhang ceilings, which is important because you have to duck to get through those places because the ceiling is so low, and so, if a fishman leaps beneath you, you take the damage but the ceiling keeps you on the platform!

Now you too know the secret knowledge driving the Fish Men.  Use it wisely.