Well, I'm certainly not.
I've just checked my collection of 'spare' banknotes and (lo & behold) of the 651 I picked out 20+ that should have been in my 'real' collection.
Nearly all of those were varieties (signatures, Series etc.) but there were about 6 that I had missed completely.
I'm just glad that I'm a hoarder and very seldom actually dispose of any of them.
Lesson learned? Don't assume or take anything for granted and double-check everything.