Grapejuice turned into vinegar immediately...

[Achmes]

Hi,

Please correct me if I'm wrong with my wine making but I was doing it a few times already and each time I was following the process of: harvesting the grapes, transporting them to the winepress, extracting the juice and IMMEDIATELY putting them into the Demijohns. After a few days I had my fermented results which I was storing in a barrel and wine lasted with no problems.

Now, yesterday I followed exactly the same process and stored the grapejuice in a Demijohn. After a few hours my liquid turned from Grapejuice into Vinegar, skipping the Wine 'status'.
It's also important to mention that my grapejuice was fresh. I know it can stale if it's left in the press for a few hours but this wasn't the case.

Can somebody experienced explain if there might be something I'm not fully aware of or it's a potential bug?

[Apocoreo]

Yesterday it turned from spring to summer. Interesting.

[Nightdawg]

The only thing I can think of is if you added a single drop of stale juice into the fresh juice by mistake, which would turn all of it into stale grapejuice. But since you just pressed the grapes, that's probably not the case.
Did you use just one Demijohn, or did it happen with multiple ones?