Malta 1965 Definitive Issue, Errors And Varieties

I am planning to record a new video for my YouTube channel about the Maltese 1965 definitive set. This set had so many errors and varieties I want to try and find out why this happened, but so far I am having no luck. There are almost 50 errors listed in SG catalogue and I know of many more unlisted varieties. Harrison & Sons were a good, well established printers so what was going on?
Any help with this topic would be gratefully appreciated.
Also any stories or unusual examples that I could use would be helpful.

Am I correct concluding most of these are missing colours? Explained in this link. They were quite common on British stamps printed by Harrison as well. The fact they occurred frequently, should not be confused with the numbers printed for each occurrence.

https://www.stanleygibbons.com/coll...d-value-flaw

Thanks NSK you are correct, they are mostly missing colours or colours printed twice/misaligned so maybe the explanation is simple after all. I'm not sure of the numbers printed but the low value of many of these errors would suggest that quite a few got through the checking process.

I came across this "variety" in my collection. The 2/- stamp on the right has a stain, like water running down the doorstep, the one on the left for comparison.
John Trory mentions a variety with "line across doorstep", could this be it? Is this just a result of over inking, or is it something else. I do not know the position or plate this comes from and don't have enough other copies, so can't research further at the moment.
Has anyone come across it before.

Since it is photogravure, it might be a cylinder and not a plate.

The right stamp has deeper colours. It might be a missing colour on the left and not a line.

Sorry I've just corrected an error in the previous post. The one on the left is the normal stamp. I have several like this all identical. The one on the right is the anomaly, with stains on the doorstep.

The video podcast has now been released. In it we discuss the various errors and varieties from the 1965 Malta definitive set and I give my thoughts on why these errors occurred.
If you want to have a look search "ourdadsstamps" on youtube.