When you enter one taxable transaction, you can't just watch the monitor. You increased your overall adjusted gross income and with that come many other changes in your return, not just the incremental tax on the one transaction. More income can make more of any Social Security taxable and can increase or decease any credits you qualified for.
By increasing your income you might have lost some credits and deductions like the EIC. I once added $6 in interest and the tax went up $12! It pushed me into the next tax bracket. I was right at the line.
