Sometimes it's a mistake, sometimes there really is a pile of power that has been contracted for, and is coming, and enough sinks that needed power which don't need any more power. There are power storage plants here and there - hydroelectric aside from small batteries. No doubt if one were on the path to this it might sink some of this power but they might be full or the grid in between the two spots might be full or they might already have their own contract in place sinking some nuclear power or who knows.
If it were on purpose I'd still expect the bid to not be so extreme. For example, maybe -5€/MWh and not the -250€ or whatever the number was. They could have still secured the sale without losing so much.
