As rotation errors on round coins are very common, I used to wonder wether such errors exist among scalloped and/or polygonal coins. A few years ago, I decided to put some extra effort in collecting circulation coins with different shapes. At some point, I bought a 15-scalloped coin turned out to have a rotation error, it is a Chatou 1918 25 Centimes: N#26410.
The red lines in the right picture show that there is a perfect 24 (360/15) degrees rotation error.
Eventhough this is quite a rare coin (Numista Rarity index: 82), I hope that someone can confirm that these coins also exist without a rotation error.
Also, I would love to know if anyone else has a scalloped or polygonal coin of any type with such a perfect rotation error, or else wether such cases have been reported before.