Suppose you own 100 shares of Coca-Cola stock in a taxable account purchased a year ago at $50 each ($5,000 cost basis). Suppose the stock's price fell to $45, so your market value is $4,500. By selling your Coca-Cola stock today at $45, you'd realize a $500 loss. If you didn't repurchase the stock within a month, you could claim the $500 loss against your capital gains; no problem. However, repurchasing the Coca-Cola stock within 30 days would trigger the wash sale rule. Repurchasing the shares within 30 days, would require that your cost basis be adjusted by the size of the loss: ($4,500 + $500 = $5,000).
What's an example of a wash sale?