It is not taxed at 25%. What you are trying to say was that in your case, 25% was withheld. It all gets rectified when you do your taxes next month, and that will determine your tax liability. Once tax liability is determined for your yearly income, you then know if you had too much withheld in which case you get a refund.

The software that does your payroll saw that extra amount of money on that check and it thinks you are going to make (as an example), 100K a year instead of 50K, so it tries to "withhold" the appropriate amount of taxes for a person making 100k.

To sum it up, you didn't get screwed.