Canada has one of the most developed paper money markets.
We collect replacements, test notes, special numbers, signatures, prefixes, changeovers, and errors. Other countries like the US and GB are just catching on to those specialties. Up until recently they only paid attention to signature varieties.
It is a mystery however, that the pricing still hasn't "matured" in such a market. Compared to the US and Australia, most Canadian paper is still cheap. Paper collecting has constantly grown in the last 15 years, more so than coins. That's why the constant rise in book values.
Errors have always been highly valued, and I've seen some interest amongst US collectors for US errors, and I'm sure other countries will follow.