First, pickled foods and fermented foods aren't the same thing. are classified by ingredients and method of preparation.

Fermented foods such as sauerkraut and fermented pickles are preserved with lactic acid. Other fresh packed or quick-packed pickled foods (such as store-bought pickles) are made with vinegar and are NOT fermented.

Lacto-fermentation is safe when properly done but it is not the same thing as what you did. Fermented pickles are made with very specific amounts of salt and water and the interaction of the 2 creates lactic acid. It is that lactic acid that makes them safe and little or no vinegar is involved.

You can find detailed information about the 3 different types of pickles - fermented, brined, and quick-pack - at NCHFP.

Store bought pickles are fresh-packed pickles and the primary ingredients of them is vinegar. The store-bought pickles are made with a specific amount of vinegar, measured for the specific amount of pickles in the recipe and for the size and shape of the cucumber chunks in that recipe. As those pickles sit in that vinegar mix the water in them is exchanged for the vinegar and the liquid in the jars is diluted/weakened. That is why they have an expiration date and must be kept refrigerated.

Now when you add fresh cucumbers, regardless of the shape, to that already weakened brine the water in those new cucumbers further dilutes the brine so what you end up with is low-acid cucumbers in a very watered down vinegar. Vinegar that is now so diluted as to be considered unable to prevent bacterial growth and so is unsafe.

OK?

Dave

PS: if interested you'll find several previous discussions here on why using used brine is not considered safe for pickles.

Here is a link that might be useful: NCHFP - Pickling

Thank you for the answer.

What if I had added more vinegar and salt at the time I put the cukes in, would it have fermented and been safe then?