Why are there so many replacement notes?

[Series B notes]

When you think of money, you typically assume it is of a higher quality than a 5 year old's hand-drawn map of Ireland. Why so many errors, then? What was up with quality control back then?

[Mac]

It is the normal amount of errors in any printing operation.
Approximately 0.5% of all bankotes on average are replacement notes. This has been determined by surveys of random populations of notes in circulation.

[ThePloughman]

Current US issues still use star notes to replace errors in production.

[Vellakare]

There are also countries which have a higher use of Banknotes per capita, particularly lower denomination notes. These tend to have a higher number of replacement notes, and in the likes of Indonesia, Thailand, Malaysia, Singapore, a more prevalent culture of paper currency collecting.

[Mac]

I miss the Irish replacement notes.
They added variety to collecting Irish notes, especially as there might still be replacement dates which haven't yet been catalogued!