A New Zealand 3-year-old girl surprised her mother by logging on to an auction site and buying a $12,000 excavator. Initially the mother thought her daughter bought a toy digger but was surprised to see the actual bill.

Pipi Quinlan, 3, woke up early one day and was able to find the TradeMe auction site on her mother’s computer. She logged on to the auction site her mother, Sarah, had browsed the previous day, searching to find toy excavators for her daughter.
Pipi managed somehow to buy an actual mechanical excavator for a sum of £8,000 ($12,400 US). The next day her mother received an email from the auction and the seller of the Kobelco excavator.
Sarah told the Rodney Times:

"I saw an email from TradeMe saying I had won an auction and another from the seller saying something like 'I think you'll love this digger'."

Initially Sarah thought she won a toy digger auction she had placed bids on the previous day but when she saw the total price of NZ$20,000 ($12,400), she was shocked.
Luckily for Sarah and Pipi, they didn’t have to pay or purchase the mechanical digger. Sarah contacted the auction site and the seller and explained what Pipi had done.
TradeMe reimbursed the seller’s cost and allowed the seller to relist the auction free of cost.

Source: Digital Journal

That's quite impressive for a 3-year-old to accomplish that.