There are two types of SS instructions: instructions with one length, from 1 to 256, and instructions with two lengths like PACK, UNPK and the packed decimal arithmetic instructions.
As others have said, it can be very easy to screw up the specification. One would certainly think that after 40+ years writing S/360 style Assembler I'd never mess this up. WRONG! Every few months I blow it, though somewhat differently, like this:
After the S0C4 I smack my forehead (again), throw out useful comments like Stupid, along with other words not to be repeated in mixed company.