Here is how I clasify all errors and stamps that were not regularly issued:
1. Overprinted specimen, "muestra"
2. Printing proofs. I still have doubts in case some proofs may have been printed on regularly issued papers and colors. The following example is printed on unwatermarked india paper, commonly used for US proofs in the 19th century.
3. Not perfed
4. Poorly perfed
5. Printed on the gum side
6. Double impresions
7. "printer's waste" and the example below seems to be, but I am not sure. It is printed on both sides on india unwatermarked paper.