Be honest. Science searches for the truth, and makes mistakes frequently. Don’t try to cover it up, instead speak to your graduate student or professor about the problem. It can be very difficult and extremely embarrassing, but “fudging” things is the near cousin of forging results, and publishing false data. If you have caused a potentially dangerous situation, by spilling a toxic substance or operating machinery improperly, do not walk away from it. Also keep in mind that your research may be published someday and contribute to the greater body of scientific knowledge and possibly affect people’s lives.
The Bottom Line: Don’t let your fear of “failure” or embarrassment in the short term get blown out of proportion and affect your research project and personal credibility in the long term. Your embarrassment will fade, but causing a major disruption or getting caught with innacurate research will not be so easy to forget.