Goblins could make a mistake regarding their loyalty in fanfiction. They are usually seen as shrewd and business - minded, but perhaps they side with the wrong wizarding faction in a conflict because they misread the political situation. Or they could make a mistake in their understanding of a new magical law. If a new law affects their traditional ways of doing business in Gringotts, they may not adapt quickly enough and end up making mistakes that lead to chaos in the wizarding economy.
In fanfiction, goblins might make a mistake when it comes to forging alliances. They could underestimate the power or intentions of a group of wizards or witches who approach them for an alliance. They might think that they can easily control the situation but end up being used by the other party. Also, goblins are known for their craftsmanship. They could make a mistake in creating a powerful magical artifact, resulting in it having unforeseen and dangerous side effects.
Well, in fanfiction, goblins might make a mistake in their financial dealings. For example, they could miscalculate the value of a magical item that a wizard or witch is trying to store in Gringotts. Maybe they misjudge the authenticity of a heirloom and accept it as collateral without realizing it's a fake. Another way could be in their security arrangements. They might let the wrong person access a certain vault due to a misidentification or a glitch in their magical security systems.