Then there are redundancies that are not pleonastic since they carry information that would not be there without the redundancy. Anu mentioned AC current, which might be thought of as redundant because the C stands for current, but without specifying current you could be talking about AC voltage. PIN number is another example since there are possible contexts in which it might not be clear if you really meant that little sharp pointy thing, or even, in some dialects, a writing implement.