It is highly unlikely for vinegar to go bad because of its acidity. the standard answer for sediment or solid material in vinegar is that this is the”mother“ a natural part of vinegar and safe.

Normal....but not always typical. Balsamic - or any wine based vinegar - can develop sediment with age. Sometimes that sediment can clump together. Or if it was not capped tightly, it could be sugar formation due to evaporation. And sometimes it will develop what is known as the 'Mother of Vinegar' or a harmless glob of mold that is used as a starter or culture for new batches of vinegar.

Other than consistency, there is almost no impact to the vinegar and it is perfectly safe to use. It has a very long shelf life :-) Strain it through some cheesecloth if you prefer.

Check the contents of your balsamic. Far too many are cheats and they add caramel and other things that are nothing to do with balsamic vinegar. I've had many a red wine or white wine vinegar make mothers but never a balsamic so I reckon you were right to throw it out, unless someone else can tell you differently.

I'm pretty sure it is the 'mother' like mentioned. Over time they grow a bit. The good balsamics from Modena, or any quality vinegars will start with a 'mother'. Mostly invisible at first bottling.

Kirkland is Costco brand. Considered one of the best balsamic vinegars for the price point.

Costco isn't good and pure acoss the entire store, but the olive oils, avocado oils and vinegars have been put under the micoscope the past few years. Lots of articles discussing their sourcing.

I'm contantly straining my AC vinegars because the 'mother' goob clogs the perforated lid cap. I decant from pantry gallons into smaller bottles kept in the cabinet above my prep area. I use a strainer when decanting but it will start forming again rather quickly.

The Costco bottles of balsamic are just too big. I don't use it that often. I use unsweetened pomegranate molasses and Italian soba in small bottles....about 1/2 a cup a year.

Yes, so many fakes across the board. The fakes are such cheaters/liars most lie about the ingredients as well. When avocado oils were tested, only Costco Chosen brand and another one were pure avocado. All others were blends and some did not have any avocado oil. Same thing happened when UCDavis tested olive oils.

I always shake the balsamic vinegar I have before using it. It does collect some dark solids at the bottom if it sits for long. I figure that's just more flavor and don't want to waste it. Balsamic isn't filtered or distilled like other vinegars are.