Scientists can as individuals, at times, be bumbling idiots just like everyone else. At times they might be dishonest.
The following lengthy quotation touches on this.
Scalia J (delivering the opinion of the Court in Melendez-Dias v Massachusetts  USSC No 07-591 (25 June 2009) (slip op pp 12 – 15), upholding the defendant's right to confront the expert and challenge his certificate of analysis:
"Forensic evidence is not uniquely immune from the risk of manipulation. According to a recent study conducted under the auspices of the National Academy of Sciences, "[t]he majority of [laboratories producing forensic evidence] are administered by law enforcement agencies, such as police departments, where the laboratory administrator reports to the head of the agency." National Research Council of the National Academies, Strengthening Forensic Science in the United States: A Path Forward 6–1 (Prepublication Copy Feb. 2009) (hereinafter National Academy Report). And "[b]ecause forensic scientists often are driven in their work by a need to answer a particular question related to the issues of a particular case, they sometimes face pressure to sacrifice appropriate methodology for the sake of expediency." Id., at S–17. A forensic analyst responding to a request from a law enforcement official may feel pressure—or have an incentive—to alter the evidence in a manner favorable to the prosecution.
"Confrontation is one means of assuring accurate forensic analysis. While it is true, as the dissent notes, that an honest analyst will not alter his testimony when forced to confront the defendant, ..., the same cannot be said of the fraudulent analyst. See Brief for National Innocence Network as Amicus Curiae 15–17 (discussing cases of documented "drylabbing" where forensic analysts report results of tests that were never performed); National Academy Report 1–8 to 1–10 (discussing documented cases of fraud and error involving the use of forensic evidence). Like the eyewitness who has fabricated his account to the police, the analyst who provides false results may, under oath in open court, reconsider his false testimony. See Coy v. Iowa, 487 U. S. 1012, 1019 (1988). And, of course, the prospect of confrontation will deter fraudulent analysis in the first place.
"Confrontation is designed to weed out not only the fraudulent analyst, but the incompetent one as well. Serious deficiencies have been found in the forensic evidence used in criminal trials. One commentator asserts that "[t]he legal community now concedes, with varying degrees of urgency, that our system produces erroneous convictions based on discredited forensics." Metzger, Cheating the Constitution, 59 Vand. L. Rev. 475, 491 (2006). One study of cases in which exonerating evidence resulted in the overturning of criminal convictions concluded that invalid forensic testimony contributed to the convictions in 60% of the cases. Garrett & Neufeld, Invalid Forensic Science Testimony and Wrongful Convictions, 95 Va. L. Rev. 1, 14 (2009). And the National Academy Report concluded:
"The forensic science system, encompassing both research and practice, has serious problems that can only be addressed by a national commitment to over-haul the current structure that supports the forensic science community in this country." National Academy Report P–1 (emphasis in original)....[footnote of Scalia J omitted]
"Like expert witnesses generally, an analyst's lack of proper training or deficiency in judgment may be disclosed in cross-examination.
" "[T]here is wide variability across forensic science disciplines with regard to techniques, methodologies, reliability, types and numbers of potential errors, research, general acceptability, and published material." National Academy Report S–5. See also id., at 5–9, 5–12, 5–17, 5–21 (discussing problems of subjectivity, bias, and unreliability of common forensic tests such as latent fingerprint analysis, pattern/impression analysis, and toolmark and firearms analysis). Contrary to respondent's and the dissent's suggestion, there is little reason to believe that confrontation will be useless in testing analysts' honesty, proficiency, and methodology—the features that are commonly the focus in the cross-examination of experts."