Assumptions

In 1 Chronicles 19, the king of the Ammonites dies, and his son succeeds him. David got along well with the previous king, and wanted to show kindness to the son. So he sent some Israelites to Ammon to express sympathy for the king's death.

Well, the new king assumes that the Israelites are spies and cuts off the seat of their robes (I'm not making this up). And he shaves them. They retreat, humiliated. And then David gets mad. But he STILL doesn't do anything to the Ammonites. They, knowing they've angered him and thinking he will attack, decide to wage war. And ultimately, they get brutally defeated by the Israelites.

All this because they assumed the worst. They looked at David's kindness and assumed it was a ploy. And they reacted to their assumptions, which caused massive consequences that could have been avoided if they'd just found out what the Israelites were really there for.

It's like the phrase goes, "Never assume, because..."