1934 King George V One Penny-Deterioration, Environment Damage Or Mint Error

I can't imagine what kind of mint error would cause that damage. It almost looks like it had been partially melted in a beer bottle cap or something. The way that the wear really builds up and presents itself as you go further to the edge makes me think it is environmental wear, even if sped up by someone doing something to the coin.

PMD of some sort that smoothed over.

Making coins is a relatively simple mechanical process; it has been highly automated in the past two centuries but the basic principle is essentially the same as it has always been: a piece of soft metal gets squashed between two harder pieces of metal. This process is well understood and well documented. All of which means that there are only a relatively small number of ways the process can go wrong, and what a coin looks like when it goes wrong in each possible way is likewise well documented.

However, once a coin leaves the mint, there are a much much larger range of possible ways that a coin could become damaged, in ways that look like they might be a mint error but are not. Such damage might happen by accident (such as tossing a coin into an industrial wood-chipper, or placing a coin underneath a heavy piece of furniture and leaving it there for a couple of decades). Or it might happen as a result of deliberate human activity (someone filing away at the edges of the coin).

Often, it's impossible to know exactly how certain types of damage have occurred - as is the case with the coin posted above. But the assumption in such cases must always be that it was not a mint error, since no definitive cause at the mint can be proven to produce such damage.

It's also simply more probable. The number of mint errors that leave the mint undetected is rally quite low, since most mints have rather good quality control in place to try to prevent that sort of thing from happening too often. The number of coins that get damaged after leaving the mint is much higher. So if you pick up a randomly selected "coin with odd damage on it", chances are is that it's not a mint error. Which is why the vast majority of the answers to people who post "is this a mint error" questions on the coin forums receive an answer "no it isn't".