Question 473045
The most common mistake is arithmetic errors, especially when multiple minus signs are involved. Or, you might accidentally substitute something wrong, e.g. -b +- sqrt(...) could accidentally be written as b +- ... (I did this in a crucial math competition once...so be careful). Any other type of mistake (e.g. reducing the denominator incorrectly) is extremely rare and shouldn't happen once you've gotten familiar with the quadratic formula.