Is the wine you found sediment in fermented in the bottle? It’s not uncommon to find organic material in hand-crafted wine.

Winecountrythisweek.com has a few explanations: “Sediment is usually an indication of a wine that was not only made many years ago but made with extraordinary care to maintain its quality and character over time. This bitter tasting but harmless residue is the byproduct of the application of little or no filtration in the winemaking process, thus enabling a wine’s personality to more fully develop in the bottle.” And, “Even white wines could leave a deposit of harmless crystals inside the bottle, which is simply an indication that the wine was previously exposed to very cold temperatures.”

These are potassium bitartrate crystals, which form when the natural tartaric acid in the wine combines with potassium under cold storage conditions. Some wines go through a process called “cold stabilization” to force these crystals to form in the winery before bottling, so they won’t form in the bottle afterward (see here).

They are completely harmless.