Post Rider With Ghost Number

I was going through some plate blocks tonight (which I no longer collect) and made an interesting discovery. This PB of #1478 shows a "ghost" plate number (34200) beneath the obvious plate number (34212), along with some doubling of the red lettering and blue denomination.

What would cause something like this in production and how common is an error like this?

It is a tagging ghost, created when ink from the previous press sheet is transferred onto the tagging mat and then transferred again from the tagging mat onto the next press sheet.

It is not common, but not rare either -- typically appearing on overall tagged US stamps of the late 1960s through the 1970s. There have been a couple of threads on SCF showing examples and explanations. It's not just the plate number that can be transferred, but any design element associated with a particular ink color might also be transferred -- resulting in a faint fuzzy shifted secondary image for anything printed of that color. An example can be found in the old thread http://goscf.com/t/9026

Nice example you have!

Here is another example from a more recent thread: http://goscf.com/t/48821

Thanks for the education! Too bad the link to that APS article is broken in the first thread. Now that I know the term for this I did a little eBay searching. Looks like you can pick up a plate block similar to mine for under $10.

You start poking around in the dark corners of your collection and there's no telling what you'll find. Would this also be an example of a tagging ghost?