I did a little research and some people thought it was a mother, from which you can make new vinegar
however depending on how much you paid for the balsamic, and if it is of a lesser quality, which does not make it bad, it just means there is more sugar in it, it may have just thickened from evaporation. If this is the case, read this.
Vinegar, in general, has a very long shelf life. Older bottles of wine-based vinegars might get sediment at the bottom, and sometimes they develop a growth of what is called Mother of Vinegar, the natural mold that is used to make new batches of vinegar. Mother looks more like a thin sheet you'll see floating in a bottle.
Mother wouldn't be as thick as the globs you're describing, though. Balsamic vinegars vary wildly in price, from the true aged balsamicos that can cost more than $100 a bottle to very cheap balsamic-flavored vinegars that cost less than $5.
With the cheap stuff, manufacturers sometimes add caramel flavoring or sweeteners to mimic the true balsamic taste. Home cooks sometimes boil a cheaper balsamic with a little sugar to make it thicker or sweeter, too.
In this case, it sounds like the vinegar thickened up from sugar. It could be that the cap wasn't tight and allowed the vinegar to evaporate.
If that's the case, it probably won't taste like it's supposed to and you'll be better off just pitching it out
A: Vinegar, in general, has a very long shelf life. Older bottles of wine-based vinegars might get sediment at the bottom, and sometimes they develop a growth of what is called Mother of Vinegar, the natural mold that is used to make new batches of vinega