What is the "Malicious Trap" in Watanagashi-hen?

Question

The manga to Watanagashi-hen (2nd question arc) ends with these words from the author (original in Japanese, translated):

About Watanagashi-hen

Malicious Trap

Watanagashi-hen, the second part following after Onikakushi-hen's scenario, focuses on explaining in greater detail about the events in Hinamizawa. Still following the once famous detective novels' style, Watanagashi-hen is the scenario that the original author himself likes the best in Higurashi no Naku Koro ni.

Noted as even more malicious than Onikakushi-hen, Watanagashi-hen is actually the mildest scenario in Higurashi no Naku Koro ni. Still, as it has been noted, there was a malicious trap by which once caught, the victim would be in great peril. Moreover, even the victim himself wouldn't notice that he had been caught in the trap.

Please confirm that in Watanagashi-hen's solution arc, Meakashi-hen.

...If by any chance did you know about being tricked into the trap... congratulations! You have enjoyed Watanagashi-hen to the fullest (LOL).

^{(Emphasis mine)}

What is the malicious trap mentioned there?

Answer 1

As it has been mentioned by the author himself, the "Malicious Trap" is exposed in the solution arc, Meakashi-hen.

A Japanese reader also asked about this on Japanese Yahoo! Answers, with additional questions that might give you hints about it.

Shion and Mion are messing up my mind... whether "Shion appearing as Mion" is a lie or not, it is very confusing. On the following cases, which girl:

1. was working at Angel Mort in the opening,
2. tortured Keiichi,
3. was being jailed,
4. stabbed Keiichi,
5. fell from the balcony?

If your answer is

1. Shion
2. Shion
3. Mion
4. Shion
5. Shion

then, "congratulations! You have enjoyed Watanagashi-hen to the fullest (LOL)."

Which then, it's clear that the malicious trap in this arc is that

Shion and Mion were switching place.

Spoiler for more details regarding the answer to above question (or as explained in Meakashi-hen):

1. The one who is working at Angel Mort is always Shion.
2. The one who tortured Keiichi was Shion. Since Shion was impersonating Mion, it's hard to know if you only see Watanagashi-hen.
3. The one who was being jailed was Mion. After that, Mion fell into the well and died because of Shion.
4. The one who stabbed Keiichi was Shion. After she ran away from the underground ritual place through the well and went back to her mansion, her Hinamizawa Syndrome worsen and reached the end stage of level 5 (the worst stage) making her insane. She climbed down the balcony, went to Keiichi's house, then stabbed Keiichi when he went outside to meet her.
5. The one who fell from the balcony was Shion. After she stabbed Keiichi, she went back to her mansion through the same balcony. While she was climbing up, her hands slipped and she fell down to her death.