Errors – Mechanical or Human ? 
The interface between man and machines, what was intended and what was done, are a fruitful source of errors. They can be broken down into general categories.
1. Human error
2. Mechanical error
3. A combination
Human error is a main source of errors. Spelling mistakes and, missing accents are common ones,. This is sometimes compounded when a second language, such as Irish or Latin is used. Another error, often unmentioned, is that of quality control. All stamps are supposedly scrutinized by humans before sale. Figure 1 shows two stamps from the 2007 €7.80 booklet. (HB142). The name is incorrectly capitalized on the stamp in row 4 position 2, but correct on all other stamps. Figure 1A shows the two in comparison.
A similar case can be made for the multi-colored cancels seen on official mail.( Figure 2). The equipment worked as designed. The problem was that someone failed to clean the machine properly.
Mechanical errors are those caused by the equipment. Paper jams, folds, wrinkles are typical of this type. Everyone did their job, but the paper still jammed. Perforation errors where a row or column is missed is highly sought after. Still, this is actually a double error. The equipment error and a failure by QC to catch it.
One specific type error is that is seen on early British stamps overprinted in Irish. The broken type gave us damaged letters and missing accents on the overprints. Is this truly a mechanical error? The type chosen for the overprints was made of a metal suitable for low volume applications, rather than hardened steel type. (Figure 3 and 3A) Why? The characters were in the Irish language which was not available in steel type. In addition, there was a tight deadline. The type metal crumpled under the pressure. Is this really a mechanical problem or poor judgement on the part of a human?
A more puzzling error is the difference in size of the logos on postal cards sent to collectors for New York Mega-Show 97 and Pacific’97. The size of the barn owl indicia on the Pacific’97 card is about 8% smaller than the one on the New York card which is the same size as the sheet stamp. Figure 4 shows the Pacific’97 logo in color overlaid on the New York logo which was artificially changed to red. Why? Apparently two different printers.
A combinations of things or events can happen to combine to cause an undesired result. The common 1d red has numerous “fly-specks”. Usually these happen because foreign matter has gotten on the plate or was on the paper. Paper dust, paper fragments and ink particles can create transient or repeatable dots or splotches. Perhaps this could have been avoided by better lighting, air flow, or cleanliness – perhaps not. Mechanical, human, or both?
The bottom line is that errors happen. It is up to the philatelist to be observant and sensitive to detail.
The Hibernian Catalog has a detailed sections on known errors on definitives and commemoratives.
The Revealer has reported various errors. A review of back issues on the searchable USB drive may prove worthwhile.