Even when you use a closed pickling or curing system, sometimes molds invade.
The first sign you have is that the delightful pickly smell we watch for fails to develop, or it develops only a day or two and then diverts to something else.
That something is first just odd, then unpleasant, and then it smells like something left in the fridge too long. This develops over many days, or sometimes weeks, and it can be difficult to tell right away. Eventually though, you KNOW, and the brine begins to cloud, and then it grays and things in it turn slimy.
We aren’t talking about some fuzz on the top which can be removed to expose good food underneath. We are talking about the stuff that goes all the way through, and that clouds the brine, and grays all the food.
It is not always technically mold, sometimes it is bacteria.
There are a few things that can cause molds in pickles.
1. Vegetables that are starting to break down can do this. Use fresh vegetables that are not showing any signs of decay.
2. Old water filters. If you use a water filter for an extended time, it can develop mold inside the filter. It is not enough to make you ill, but it CAN be enough to start some real nasties in a curing environment where it can get a good foothold. The solution here is to either use the water unfiltered (it will have some chlorine, but our pickles have been successful anyway), OR boil the water and let it cool to room temp, and then start the cure. Do not use the water hot or very warm, it tends to encourage the wrong things.
3. Too little salt. Use full salt ratios to avoid the growth of molds throughout the food.
These foods were made in a non-sterile environment for thousands of years. This is how they were developed, so failure to sterilize things is pretty much never the cause of a pickling failure. It has to be something with a fairly large amount of bacteria or fungus added.
It is terribly disappointing to put the effort into starting the production of a good food, only to have it spoiled and inedible. The loss of the investment in the food is also a thing we feel.
If your ancestors could create these things in a crock in a dirt floored root cellar, though, we should be able to create them reliably in a good jar under better controlled conditions.