I got in a bundle of 5$ notes taken from a brick many (32 out of 100) notes having this small curious-looking printing error: the children sliding in the toboggan at the left on the back of the note seems to have some snow between legs... look at the scan below. There is a white patch just at this place, instead of the usual blue ink.
And this error was repeating every three notes, with some exceptions however. The notes between were normal. Here the serial numbers having the variations: HON7238916, 913, 910, 907, 902, 899, 896, 893 , 890, 887, 884, 881, 878, 875, 872, 868, 865, 862, 859, 856, 853, 850, 847, 844, 841, 838, 835, 832, 829, 828, 825, 824. The back position number is 19 (fpn = 18). I have some difficuties to explain thi pattern. If it's a defect in the plate # 19, why it's was not found in every note? And if it's caused by an interposing substance, why it repeats quite regularly and always at the same place? Any suggestion?